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UNIVERSITY  OF  CALIFORNIA 
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"  JUL  1  3  'W 

7/f^   STORY  OF   THE   STARS. 


NEW 


DESCRIPTIVE 


ASTRONOMY, 


JOEL    DORM  AN    STEELE,    Ph.D., 

AUTHOR    OF   THE    FOURTEEN-VVEEKS   SERIES   IN    NATURAL   SCIENCE. 


' '  The  heavens  cteolare  the  glory  of  God;  and  the   fi'nmament  showezn 
is  handiwork.  " — PsAlM  XIX,  1. 


Copyright,  1869,  1884,  by 

A.     S.     BARNES     &     COMPANY, 
NEW   YORK   AND    CHICAGO. 


A    POPULAR    SERIES 

IN 

NATURAL    SCIENCE, 

J.   DoRNiAN    Steele,    PH.n.,    K.G.S., 

A  uihor  o/ the  Fourteen  Weeks  Series  in  Natural  Science,  etc.,  etc. 

New  Popular  Chemistry.  New  Descriptive  Astronomy. 

New  Popular  Physics.  New  Hygienic  Physiology. 

New  Popular  Zoology.  Popular  Geology. 

An  Introduction  to  Botany. 

The  Publishers  can  supply  (to  Teachers  only)  a  Key  containing  Answers  to  the 
Questions  and  Problems  in  Steele's  entire  Series. 


BARNES'     HISTORICAL     SERIES, 

ON     THE     FLAN      OK 

STEELE'S     FOURTEEN-WEEKS     IN     THE     SCIENCES. 

A  Brief   History  of  the   United   States. 
A  Brief  History  of  France. 

A  Brief  History  of  Ancient   Peoples. 

A   Brief   History   of   Mediaeval   and    Modern    Peoples^ 
A   Brief  General    History. 

A  Brief  History  or  Greece. 

A  Brief  History  of  Rome. 

A  Popular  History  of  the  United  States. 


Library         S  '^Z  S 


PREFACE  TO  THE  FIRST  EDITION. 


~r~\URING  the  past  few  years  great  advances 
-■-^  have  been  effected  in  astronomical  science. 
Physics  has  come  to  the  help  of  Mathematics,  and, 
not  content  with  the  old  question,  where  the  heav- 
enly bodies  are,  has  sought  to  find  out  what  they  are. 
Valuable  discoveries  have  been  made  concerning 
Meteors,  Shooting  Stars,  the  Constitution  of  the  Sun, 
the  Motion  of  the  Heavenly  Bodies,  &c.  The  investi- 
gations connected  with  Spectrum  Analysis  have  been 
especially  suggestive.  On  every  hand  the  facts  of 
the  New  Astronomy  have  been  accumulating.  Until 
recently,  however,  they  were  scattered  through  many 
expensive  books,  and  were  consequently  beyond  the 
reach  of  the  most  of  our  schools.  It  has  been  the 
aim  to  collect  in  this  little  volume  the  most  interest- 
ing features  of  the  larger  works. 

Believing  that  Natural  Science  is  full  of  fascina- 
tion, the  author  has  sought  to  weave  the  story  of 
those  far-distant  worlds  into  a  form  that  may  attract 
the  attention  and  kindle  the  enthusiasm  of  the  pupil. 

This  work  is  not  written  for  the  information  of 
scientific  men,    but    for  the  inspiration   of    youth. 

20482'? 


VI  PREFACE  TO   THE  FIRST  EDITION. 

Therefore  the  pages  are  not  burdened  with  a  multi- 
tude of  figures  which  no  memory  could  retain. 

Mathematical  tables  and  data,  Questions  for  Re- 
view, a  very  valuable  Guide  to  the  Constellations, 
and  an  Apparatus  for  Illustrating  Precession,  are 
given  in  the  Appendix,  where  they  may  be  useful 
for  reference. 

Those  persons  having  a  small  telescope  will  find 
valuable  assistance  in  the  ''  List  of  interesting  Ob- 
jects for  a  common  Telescope."  The  Index  contains 
the  pronunciation  of  many  difficult  names. 

Particular  attention  is  called  to  the  method  of 
classifying  the  measurements  of  Space,  and  the 
practical  treatment  of  the  subjects  of  Parallax,  Har- 
vest Moon,  Eclipses,  the  Seasons,  Phases  of  the  Moon, 
Time,  Nebular  Hypothesis,  Spectrum  Analysis,  and 
Precession. 

To  teachers  hitherto  compelled  to  use  a  cumber- 
some set  of  charts,  it  is  hoped  that  the  star  maps 
here  offered  will  present  a  welcome  substitute.  The 
geometrical  figures,  showing  the  actual  appearance  of 
the  constellations,  will  relieve  the  mind  confused  with 
the  idea  of  numberless  rivers,  serpents,  and  classical 
heroes.  Only  the  brightest  stars  are  given,  since  in 
practice  it  is  found  that  pupils  remember  the  general 
outlines  alone,  while  the  details  are  soon  forgotten. 

Many  of  the  cuts  are  copied  from  the  French  edi- 
tion  of  Guillemin's  '^  Heavens."    Acknowledgment 


ASTRONOMY.  vil 

for  much  valuable  material  is  hereby  made  to  this 
excellent  work,  and  also  to  "Chambers's  Astron- 
omy," "Newcomb's  Astronomy,"  and  Young's  "The 
Sun." 

Finally,  the  author  commits  this  little  work  to  the 
hands  of  the  young,  to  whose  instruction  he  has  con- 
secrated the  energies  of  his  life,  in  the  earnest  hope 
that,  loving  Nature  in  all  her  varied  phases,  they 
may  come  to  understand  somewhat  of  the  wisdom, 
power,  beneficence,  and  grandeur  displayed  in  the 
Divine  harmony  of  the  Universe. 

"One  God,  one  law,  one  element, 

And  one  far-off  Divine  event 
To  which  the  whole  creation  moves. " 


READING    REFERENCES. 

Chambers's  Astronomy. — Young's  The  Sun.— Ball's  Elements  of  Astronomy.— 
Newcomb's  Popular  Astronomy. — Lockyer's  Spectrum  Analysis. — Proctor's  Other 
Wbrlds  than  Ours,  Saturn,  The  Moon,  Poetry  of  Astronomy,  &c. — Delaunay's  Cours 
D'Astronomie. — Haughton's  Manual  of  Astronomy. — Newcomb  and  Holden's  As- 
tronomy.— Lockyer's  Elements  of  Astronomy. — Norton's  Spherical  and  Physical 
Astronomy.  —  Herschel's  Outlines  of  Astronomy. — Robinson's  Astronomy, — Mitch- 
ell's Popular  Astronomy.  —  Arago's  Popular  Astronomy.  —  Airy's  Lectures  on 
Astronomy. — Hind's  Solar  System,  and  Introduction  to  Astronomy. — Lockyer's  Ele- 
mentary Lessons  in  Astronomy.— Proctor's  Star  Atlas. — Heis's  Star  Atlas. — Peck's 
Popular  Astronom}-. — Gillet  and  Rolfe's  Astronomy.- Sharpless  and  Phillips's  As- 
tronomy.— Peabody's  Elements  of  Astronomy. — Schellen's  Spectrum  Analysis. — 
Winchell's  World-Life  (excellent  reading  in  connection  with  the  Nebular  Hypothe- 
sis).—Flammarion's  Wonders  of  the  Heavens.— Guillemin's  The  Heavens,  revised  by 
Proctor. — Loomis's  Elements  of  Astronomy. — Proctor's  Easy  Star  Lessons. — Olm- 
stead's  Letters  on  Astronomy. — Routledge's  Historj' of  Science. — Buckley's  History 
of  Natural  Science. — Williamson's  Problems  on  the  Globes. — The  Popular  Science 
Monthly  (1872-1884;. — Rambosson's  Histoire  Des  .Astres. 


SUGGESTIONS    TO    TEACHERS. 


^l"^HIS  work  is  designed  to  be  recited  in  the  topical  method.  On  hear- 
ing the  title  of  a  paragraph,  the  pupil  should  be  able  to  draw  upon 
the  blackboard  the  diagram,  and  to  state  the  substance  of  what  is  con- 
tained in  the  book.  It  will  be  noticed  that  the  order  of  topics,  in  treat- 
ing of  the  planets  and  also  of  the  constellations,  is  uniform.  If,  each 
day,  a  portion  of  the  class  write  their  topics  in  full  upon  the  black- 
board, it  will  be  found  a  valuable  exercise  in  spelling,  punctuation, 
and  composition.  Every  point  which  can  be  illustrated  in  the  heavens 
should  be  shown  to  the  class.  No  description  or  apparatus  can  equal 
the  reality  in  the  sky.  After  a  constellation  has  been  traced,  the  pupil 
should  be  practised  in  star-map  drawing. 

The  article  on  "Celestial  Measurements,"  near  the  close  of  the 
work,  should  be  constantly  referred  to  during  the  term.  In  the  figures, 
and  especially  in  the  star-maps,  it  should  be  remembered  that  the 
right-hand  side  represents  the  west  ;  and  the  left-hand,  the  east.  To 
obtain  this  idea  correctly,  the  book  should,  in  general,  be  held  up 
toward  the  southern  sky. 

For  the  purpose  of  more  easily  finding  the  heavenly  bodies  at  any 
time,  Whitall's  Movable  Planisphere  is  of  great  service.  It  may  be 
procured  of  the  publishers  of  this  work.  A  tellurian  is  invaluable  in 
explaining  Precession  of  the  Equinoxes,  Eclipses,  Phases  of  the  Moon, 
etc.  Messrs.  A.  S.  Barnes  &  Co. ,  New  York  City,  furnish  a  good  in- 
strument at  a  low  price.  A  small  telescope,  or  even  ah  opera-glass, 
will  be  useful.  A  good  star-map,  and  as  many  advanced  works  upon 
Astronomy  as  can  be  secured,  should  be  included  in  the  teacher's 
outfit. 


X  SUGGESTIONS  TO  TEACHERS. 

The  pupil  should,  at  the  outset,  get  a  distinct  idea  of  the  circles 
and  planes  of  the  celestial  sphere.  The  subject  of  angular  measure- 
ments can  easily  be  made  clear  in  this  relation.  A  circle  contains 
360°  ;  90"  reach  from  horizon  to  zenith  ;  180"  produce  opposition  ; 
while  smaller  distances  can  be  shown  in  the  sky  (see  pp.  216,  228). 

Never  let  a  pupU  recite  a  lesson,  nor  answer  a  question,  except  it  be 
a  mere  definition,  in  the  language  of  the  book.  The  text  is  designed  to 
interest  and  instruct  the  pupil  ;  the  recitation  should  afford  him  an 
opportunity  of  expressing  what  he  has  learned,  in  his  own  style  and 
words. 

Teachers  desiring  additional  information  are  advised  to  read  "  New- 
comb's  Astronomy,"  Young's  "The  Sun,"  Proctor's  Works,  "Chambers's 
Astronomy,"  and  Ball's  "  Elements  of  Astronomy." 


TABLE    OF    CONTENTS 


PAGE 

INTRODUCTORY  REMARKS 1 


I.    INTRODUCTION. 

HISTORY  OF  ASTRONOMY 5 

SPACE 24 

The  Thkee  Systems  op  Circles 36 

The  Zodiac 31 

II.    THE    SOLAR    SYSTEM 35 

THE  SUN 36 

THE  PLANETS 55 

Vulcan 71 

Mercury 71 

Venus 77 

The  Earth 82 

The  Seasons 95 

Precession  and  Nutation 104 

Refraction,  Aberration,  and  Parai,lax 112 

The  Moon 122 

Eclipses 138 

The  Tides 147 

Mars 150 

The  Minor  Planets 154 

Jupiter 157 

Saturn 164 

Uranus 170 

Neptune 172 


Xll  TABLE   OF   CONTENTS. 

PAGE 

METEORS   AND  SHOOTING   STARS 175 

COMETS 185 

ZODIACAL  LIGHT 196 


III.    THE    SIDEREAL    SYSTEM 201 

THE  STARS 203 

THE  CONSTELLATIONS 214 

Northern  Circumpolar  Constellations 214 

Equatorial  Constellations 220 

Southern  Constellations 238 

DOUBLE    STARS,    COLORED    STARS,    VARIABLE    STARS, 

CLUSTERS,  MAGELLANIC  CLOUDS,  &C 239 

Nebula 246 

The  INIiLKY  Way 253 

The  Nebular  Hypothesis 255 

CELESTIAL  CHEMISTRY.— Spectrum  Analysis 258 

TIME 263 

CELESTIAL  MEASUREMENTS 271 

IV.    APPENDIX 289 

• 

Tables 291 

Questions 293 

Guide  to  the  Constellations 313 

Apparatus 317 

List  of  Interesting  Objects  Visible  with  an  Ordinary 

Telescope 319 

Index  323 


INTRODUCTORY    REMARKS* 

A  STROif  OM  Y  (astron,  a  star ;  nomos,  a  law)  treats  of  the 
/-\  Heavenly  Bodies — the  sun,  moon,  planets,  stars,  etc., 
and,  as  our  globe  is  a  planet,  of  the  earth  also.  It 
is,  above  all  others,  a  science  that  cultivates  the  imagination. 
Yet  its  theories  and  distances  are  based  upon  rigorous  matliemat- 
ical  demonstrations.  Thus  the  study  has  at  once  the  beauty  of 
poetry  and  the  exactness  of  Geometry. 

The  great  dome  of  the  sky,  lilled  with  glittering  stars,  is  one 
of  the  most  sublime  spectacles  in  nature.  To  enjoy  this  fully,  a 
night  must  be  chosen  when  the  air  is  clear,  and  the  moon  is  ab- 
sent. We  then  gaze  upon  a  deep  blue,  an  immense  expanse 
studded  with  stars  of  varied  color  and  brilliancy.  Some  shine 
with  a  vivid  light,  perpetually  changing  and  twinkling  ;  others, 
more  constant,  beam  tranquilly  and  softly  upon  us  ;  while  many 
just  tremble  into  our  sight,  like  a  wave  that,  struggling  to  reach 
some  far-off  land,  dies  as  it  touches  the  shore. 

In  the  presence  of  such  weird  and  wondrous  beauty,  the  ten- 
derest  sentiments  of  the  heart  are  aroused.  A  feeling  of  awe  and 
reverence,  of  softened  melancholy  mingled  with  a  thought  of 
God,  comes  over  us,  and  awakens  the  better  nature  within  us. 
Those  far-off  lights  seem  full  of  meaning  to  us,  could  Ave  but 
read  their  message  ;  they  become  real  and  sentient,  and,  like  the 
soft  Cj  es  in  pictures,  look  lovingly  and  inquiringlj'^  upon  us.  "We 
come  into  communion  with  another  life,  and  the  soul  asserts  its 
immortality  more  strongly  than  ever  before.  We  are  humbled 
as  we  gaze  upon  the  infinity  of  suns,  and  strive  to  comprehend 

*  This  Introduction  is  designed  merely  to  furnish  su{;gestive  material  for  conversa- 
tion at  the  first  lesson,  preparatory  to  beginning  the  study.  It  is  not  intended  for  com- 
mittal.   Other  topics  may  he  found  in  the  Questions  given  in  the  Appendix. 


2  INTRODUCTORY  REMARKS. 

their  enormous  distances,  and  their  magnificent  retinue  of  worlds. 
The  powers  of  the  mind  are  aroused,  and  eager  questionings 
crowd  upon  us.  What  are  those  glittering  fires  1  WTiat  is  their 
distance  1  Are  they  worlds  like  our  own  ?  Do  living,  thinking 
beings  dwell  upon  them  ?  Are  they  promiscuously  scattered 
tlirough  space,  or  is  there  a  system  in  the  universe  ?  Can  we 
ever  hope  to  fathom  those  mysterious  depths,  or  are  they  closed 
to  us  forever  1 

Some  of  these  problems  have  been  solved ;  others  yet  await 
the  astronomer  whose  eye  shall  be  keen  enough  to  read  the  mys- 
terious scroll  of  the  heavens.  Two  hundred  generations  of  study 
have  revealed  to  us  such  startling  facts,  that  we  Avonder  how  man 
in  liis  feebleness  can  grasp  so  much,  see  so  far,  and  penetrate  so 
deeply  into  the  mysteries  of  the  universe.  Astronomy  has  meas- 
ured the  distance  of  a  few  stars,  and  of  all  the  planets ;  com- 
puted the  mass,  size,  days,  years,  seasons,  and  many  physical 
features  of  the  planets ;  made  a  map  of  the  moon ;  tracked 
many  of  the  comets  in  their  immense  sidereal  journeys  ;  and,  at 
last,  analyzed  the  structure  of  the  sun  and  stars,  and  announced 
the  very  elements  of  wliich  they  are  composed. 

Observing  for  several  evenings  those  stars  which  shine  with  a 
clear,  steady  light,  we  notice  that  they  change  their  position  with 
respect  to  the  others.  They  are  therefore  called  planets  (literally 
wanderers).  Others  remain  immovable,  and  shine  with  a  shift- 
ing, twinkling  light.  They  are  termed  the  fixed  stars,  although 
it  is  now  knoAvn  that  they  also  are  in  motion — the  most  rapid  of 
any  known  even  to  Astronomy — but  through  such  immense 
orbits  that  they  seem  to  us  to  be  stationary.  Then,  too,  diag- 
onally girdling  the  heavens,  is  a  whitish,  vapory  belt — the  Milhy 
Wmj.  This  is  composed  of  multitudes  of  millions  of  suns — of 
which  our  OAvn  sun  itself  is  one — so  far  removed  from  us  that 
their  light  mingles,  and  makes  only  a  fleecy  whiteness. 

Tliis  magnificent  panorama  of  the  heavens  is  before  us,  inviting 
our  study,  and  waiting  to  make  known  to  us  the  grandest  revela- 
tions of  science. 


I. 
INTRODUCTION 


(   1.  Among  the  Chinese. 

2.  Among  the  Chaldeans. 

(\.  Thales. 

1  2.  Anaximander. 

3.  Among  the  Grecians.  ■{  3.  Pythagoras. 

I  4.  Auaxagoras  <fe  Eudoxus. 
V  5.  Hipparchus. 

„       ^  (1.  The  School  at  Alexandria. 

4.  The  Egyptians -^  j,    ptokiny  and  his  Theory. 


1.  History 


O 

l-H 

o 


2.  Space. 


5.  The  Saracens. 

6.  Astrology. 

7.  The  C.opernican  System. 
S.  Tycho  Braheu 

9.  Kepler's  Laws. 

(  1.  His  Telescope, 

10.  Galileo \  2.  His  Discoveries. 

(  3.  Their  Reception. 


Newton,  and  the  Law 
OF  Gravitation 


{a. 
6. 
c. 


a.  Laws  of  Motion. 

Their  Application  to  Moon's 

Pha.ses. 
Tlie  Result 


1.  Celestial  Sphere. 


The  Three  Sys- 
tems OF  Cir- 
cles. 


'^   3.  The  Zodiac. 


a.  The  Principal  Circle. 

1.  The  Hori-  )  6.  Tlie  Subord.  Circle, 
zon.         l  c.  Points. 

d.  Measurements. 

(  a.  The  Principal  Circle. 

2.  The  Equi-  1  6.  The  Subord.  Circle. 


noctial. 


3.  The 

Ecliptic. 


Points. 
[  d.  Measurements. 

a.  The  Principal  Circle. 
6.  The  Subord.  Circle. 
r.  Points. 
d.  Measurements. 


Fig.  S. 


Sir  Isaac  Newtcm. 


L-THE    HISTORY. 

Astronomy  is  the  most  ancient  of  the  sciences. 
The  study  of  the  stars  is  doubtless  as  old  as  man 
himself,  and  hence  many  of  its  discoveries  date  back 
of  authentic  records,  amid  the  mysteries  of  tradition. 
In  tracing  its  history,  we  shall  speak  only  of  those 


6  THE  HISTORY. 

prominent  facts  that  will  enable  us  to  understand  its 
progress  and  glorious  achievements. 

The  Chinese  boast  much  of  their  astronomical  dis- 
coveries. Indeed,  their  emperor  claims  a  celestial 
ancestry,  and  styles  himself  the  Son  of  the  Sun. 
They  possess  an  account  of  a  conjunction  of  four 
planets  and  the  moon,  which  occurred  in  the  25th 
century  before  Christ.  They  have  also  the  first  record 
of  an  eclipse  of  the  sun  (b.c.  2128) ;  and  one  of  their 
emperors  put  to  death  the  chief  astronomers  Ho  and 
Hi  for  failing  to  announce  the  solar  eclipse  of 
2169  B.C. 

The  Chaldeans. — The  Chaldean  shepherds,  watch- 
ing their  flocks  by  night  under  a  sky  famed  for  its 
clearness  and  brilliancy,  could  not  fail  to  become 
familiar  with  many  of  the  movements  of  the  heav- 
enly bodies.  Their  priests  were  astronomers ;  and 
their  temples,  observatories.  When  Alexander  took 
Babylon  (b.c.  331),  he  found  a  record  of  their  obser- 
vations reaching  back  nineteen  centuries.*  The 
Chaldeans  divided  the  day  into  hours,  invented  the 
sun-dial,  and  discovered  the  Saros,  or  Chaldean  Pe- 
riod— the  length  of  time  in  which  eclipses  of  the  sun 
and  the  moon  repeat  themselves  in  the  same  order. 

The  Grecians. — Though  the  Asiatics  were  patient 
observers,  they  did  not  classify  their  knowledge,  and 
lay  the  basis  of  a  science.  This  became  the  work  of 
the  western  mind. 

Thales  (B.C.  640-548),  one  of  the  seven  sages  of 

*  Many  astronomical  inscriptions  have  been  found  in  the  ruins  of  Nineveh.  In  the 
public  library  of  that  city  there  was  a  series  of  about  se^'enty-two  A'olunies,  called  the 
Observations  of  Bel.  One  book  treated  of  the  polar  star  (then  Alpha  of  the  Dragon), 
another  of  Venus,  and  a  third  of  Mars.  Tlie  earliest  of  these  records  are  thought  tO 
date  back  as  fiw  as  2540  b.c.    (See  Kecords  of  the  Past,  Vol.  I.) 


THE  GRECIANS.  7 

Greece,  has  been  styled  the  Father  of  Astronomy. 
He  taught  that  the  earth  is  round,  and  that  the  moon 
receives  her  light  from  the  sun.  He  determined 
when  the  equinoxes  and  the  solstices  occur,  and  also 
predicted  an  eclipse  of  the  sun  that  is  famous  for 
having  terminated  a  war  between  the  Medes  and  the 
Lydians.  These  nations  were  engaged  in  a  fierce 
battle,  but  the  awe  produced  by  the  darkening  of  the 
sun  was  so  great,  that  both  sides  threw  down  their 
arms  and  made  peace. 

Anaximander  (B.C.  610-546)  invented  the  sun-dial, 
and  explained  the  cause  of  the  moon's  phases. 

Pythagoras  (b.c.  570-500)  founded  a  celebrated 
astronomical  school  at  Crotona,  Italy,  where  were 
educated  hundreds  of  enthusiastic  pupils.  *  He  was 
emphatically  a  dreamer.  He  conceived  a  system  of 
the  universe,  in  many  respects  correct ;  yet  he  ad- 
vanced no  proof,  made  few  converts  to  his  views, 
and  they  were  soon  well-nigh  forgotten. 

He  held  that  the  sun  is  the  center  of  the  solar  sys- 
tem, the  planets  revolving  about  it  in  circular  orbits  ; 
that  the  earth  rotates  daily  on  its  axis,  and  revolves 
yearly  round  the  sun ;  that  Venus  is  both  morning 
and  evening  star ;  that  the  planets  are  placed  at 
intervals  corresponding  to  the  scale  in  music,  and 
that  they  move  in  harmony,  making  the  '*  music  of 
the  spheres,"  but  that  this  celestial  concert  is  heard 
only  by  the  gods, — the  ears  of  man  being  too  gross 
for  such  divine  melody.  He  also  believed  that  the 
planets  are  inhabited,  and  he  even  attempted  to  cal- 
culate the  size  of  the  animals  in  the  moon. 

*  §ee  Bjimes's  History  of  the  Ancient  Peoples,  p.  17*. 


8  THE  HISTORY. 

Anaxagoras  (B.C.  500-428)  taught  that  there  is  but 
one  God,  and  that  the  sun  is  only  a  fiery  globe,  and 
should  not  be  worshipped.  He  attempted  to  explain 
eclipses  and  other  celestial  phenomena  by  natural 
causes,  saying  that  there  is  no  such  thing  as  chance 
or  accident,  these  being  only  names  for  unknown 
laws.  For  his  audacity  and  impiety,  as  his  country- 
men considered  it,  he  and  his  family  were  doomed  to 
perpetual  banishment. 

EuDOXUS,  who  lived  in  the  fourth  century  B.C.,  in- 
vented the  theory  of  the  Crystalline  Spheres.  He 
held  that  the  heavenly  bodies  are  set,  like  gems,  in 
hollow,  transparent,  crystal  globes,  which  are  so 
pure  that  they  do  not  obstruct  our  view,  while  they 
all  revolve  around  the  earth  ;  and  that  the  planets 
are  placed  in  one  globe,  but  have  a  power  of  moving 
themselves,  under  the  guidance — as  Aristotle  taught 
— of  a  tutelary  genius,  who  resides  in  each,  and  rules 
over  it  as  the  mind  rules  over  the  body. 

HiPPARCHUS,  who  flourished  in  the  second  century 
B.C.,  has  been  called  the  Newton  of  Antiquity.  He 
was  the  most  celebrated  of  the  Greek  astronomers. 
He  calculated  the  length  of  the  year  to  within  six  min- 
utes, discovered  the  precession  of  the  equinoxes,  and 
made  the  first  catalogue  of  the  stars — 1080  in  number. 

The  Egyptians. — Egypt,  as  well  as  Chaldea,  was 
noted  for  its  knowledge  of  the  sciences  long  before 
they  were  cultivated  in  Greece.  It  was  the  practice 
of  the  Greek  philosophers,  before  aspiring  to  the 
rank  of  teacher,  to  travel  for  years  through  these 
countries,  and  gather  wisdom  at  its  fountain-head. 
Pythagoras  spent  thirty  years  in  this  kind  of  study. 


THE  EGYPTIANS.  9 

Two  hundred  years  after  Pythagoras,  the  cele- 
brated school  of  Alexandria  was  established.*  Here 
were  concentrated  in  vast  libraries  and  princely 
halls  nearly  all  the  wisdom  and  learning  of  the 
world.  Here  flourished  the  sciences  and  arts,  under 
the  patronage  of  munificent  kings. 

At  this  school,  Ptolemy  (a.d.  70),  a  Grecian,  wrote 
his  great  work,  the  Almagest,  which  for  fourteen 
centuries  was  the  text-book  of  astronomers.  In  this 
work  was  given  what  is  known  as  the  Ptolemaic 
System.  It  was  founded  largely  upon  the  materials 
gathered  by  previous  astronomers,  such  as  Hippar- 
chus,  whom  we  have  already  mentioned,  and  Era- 
tosthenes, who  computed  the  size  of  the  earth  by  the 
means  even  now  considered  the  best — the  measure- 
ment of  an  arc  of  the  meridian. 

Ptolemaic  System. — To  the  early  astronomers,  the 
movements  of  the  planets  seemed  extremely  com- 
plex. Venus,  for  instance,  was  sometimes  seen  as 
evening  star  in  the  west,  and  then  again  as  morning 
star  in  the  east.  Sometimes  she  appeared  to  be 
moving  in  the  same  direction  as  the  sun,  then,  going 
apparently  behind  the  sun,  she  seemed  to  pass  on 
again  in  a  course  directly  opposite.  At  one  time, 
she  would  recede  from  the  sun  more  and  more  slowly 
and  coyly,  until  she  would  appear  to  be  entirely  sta- 
tionary ;  then  she  would  retrace  her  steps,  and  seem 
to  meet  the  sun. 

An  attempt  was  made  to  account  for  all  "these 
facts  by  an  incongruous  system  of    ''  Cycles  and 

*  See  Barnes's  General  History,  p.  154. 


10 


THE  HISTORY. 


epicycles,*'  as  it  is  called.*  The  advocates  of  this 
theory  assumed  that  every  planet  revolves  in  a 
circle,  and  that  the  earth  is  the  fixed  center  around 
which  the  sun  and  the  heavenly  bodies  move.  They 
then  conceived  that  a  bar,  or  something  equivalent, 
is  connected  at  one  end  with  the  earth  ;  that  at  some 
part  of  this  bar  the  sun  is  attached ;  while  between 
that  and  the  earth,  Venus  is  fastened — not  to  the  bar 
directly,  but  to  a  sort  of  crank  :  and  further  on,  Mer 
cury  is  hitched  on  in  the  same  way. 

In  Fig.  3,  let  A  be  the  earth  :  S,  the  sun  :  A  B  D  F, 
the  bar  (real  or  imaginary) ;  B  C,  the  short  bar  or 
crank  to  which  Venus  is  tied  ;  D  E,  another  bar  for 
Mercury  ;  F  G,  a  fourth  bar.  with  still  another  short 
crank,  at  the  end  of  which,  H,  Mars  is  attached. 


Fig.  3. 


The  Ptolemaic  System, 

Thus  they  had  a  complete  system.  They  did  not 
exactly  understand  the  nature  of  these  bars — 
whether  they  were  real  or  only  imaginary — but  they 
did  comprehend  their  action,  as  they  thought ;  and 

•  Hilton  refers  to  this  when  he  speaks  of  the  heavens  as — 

"  With  centric  and  eccentric  scribbled  o'er, 
Cycle  and  epicycle,  orb  in  orb." 


THE   SARACENS.  ll 

SO  they  supposed  the  bar  revolved,  carrying  the  sun 
and  planets  along  in  a  large  circle  about  the  earth  ; 
while  all  the  short  cranks  kept  flying  around,  thus 
sweeping  each  planet  through  a  smaller  circle. 

By  this  theory,  we  can  see  that  the  planets  would 
sometimes  go  in  front  of  the  sun  and  sometimes 
behind ;  and  their  places  were  so  accurately  pre- 
dicted, that  the  error  could  not  be  detected  by  the 
rude  instruments  then  in  use.  As  soon  as  a  new 
motion  of  one  of  the  heavenly  bodies  was  discovered, 
a  new  crank,  and  of  course  a  new  circle,  was  added 
to  account  for  the  fact.  Thus  the  system  became 
more  and  more  complicated,  until,  at  last,  a  coip.- 
bination  of  five  cranks  and  circles  was  necessary  to 
make  the  planet  Mars  keep  pace  with  the  Ptolemaic 
theory.  No  wonder  that  Alfonso,  of  Castile,  a 
celebrated  patron  of  Astronomy,  revolted  at  the 
cumbersome  machinery,  and  cried  out,  "  If  I  had 
been  consulted  at  the  Creation,  I  could  have  done 
the  thing  better  than  that." 

The  Saracens.  —  After  the  destruction  of  the 
library  at  Alexandria,  learning  found  a  home  among 
the  Mohammedans.  Bagdad  on  the  Tigris,  and  Cor- 
dova on  the  Guadalquiver  became  centers  of 
science,  literature,  and  art.  The  treasures  of  Grecian 
knowledge  were  eagerly  gathered  by  the  Caliphs, 
and  we  are  told  that  it  was  not  uncommon  to  see, 
entering  the  gates  of  Bagdad,  a  whole  train  of 
camels  loaded  with  Greek  manuscripts.  Gerbert, 
afterward  Pope  Sylvester  II.,  learned  the  elements 
of  astronomy  at  the  University  of  Cordova,  going, 
after  the  custom  of  the  time,  to  Spain  for  instruc- 


12 


THE  HISTORY. 


Fig.  U. 


tion,  as,  formerlY,  philosophers  had  gone  to  Egypt 
In    the   Moorish    schools,   geography    was    already 
taught  by  the  use  of  the  globe.     The  first  observ- 
atory in  Europe  was  erected  at 
Seville    (1196).       The    fragments 
of  Saracenic  learning  that  have 
come     down    to    us    show    that 
the  Arabs  had  constructed  astro- 
nomical   tables,   and  endeavored 
to  perfect  them  by  means  of  sys- 
tematic observation  of  the 
heavens.  With  the  down- 
fall   of    the    Moors,    and 
the  Revival  of  Learning, 
Spain  ceased  to  take  the 
lead  in  scientific  study. 


iVi*  {Jiraiila,  Muorish  OO^ 


ASTROLOGY.  13 

Astrology. — During  all  these  centuries,  astronomy 
owed  its  development  quite  as  much  to  a  desire  of 
foretelling  the  future,  as  to  a  love  for  science.  It 
was  the  prevalent  belief  that  the  stars  rule  the  des- 
tinies of  men.  The  Chaldeans  scanned  the  heavens 
for  purposes  of  divination,  so  that  Chaldean  and 
astrologer  became  synonymous.  Tiberius,  Emperor 
of  Rome,  practised  astrology.  Hippocrates  himself, 
the  Father  of  Medicine  (b.c.  470),  ranked  this  among 
the  most  important  branches  of  knowledge  for  the 
physician.  The  mysterious  study  possessed  a  pecu- 
liar fascination  for  the  Arabians,  and  they  culti- 
vated it  assiduously.  The  Moorish  astronomers  were 
astrologers  as  well,  and  popularized  the  art  in  west- 
ern Europe.  This  superstition  reached  the  height  of 
its  influence  during  the  Middle  Ages. 

The  issue  of  any  important  undertaking  and  the 
fortunes  of  an  individual  were  foretold  by  the  as- 
trologer, who  drew  up  a  Horoscope  representing  the 
position  of  the  sun,  moon,  and  planets  at  the  begin- 
ning of  the  enterprise,  or  at  the  birth  of  the  person. 
It  was  a  complete  and  complicated  system,  and  con- 
tained regular  rules,  which  guided  the  interpretation, 
and  which  were  so  abstruse  as  to  require  years  for 
their  mastery.  Venus  foretold  love ;  Mars,  war ; 
the  Pleiades  (Ple'-ya-dez),  storms  at  sea. 

The  ignorant  were  not  the  only  dupes  of  this 
visionary  system.  Lord  Bacon  believed  in  it  most 
firmly.  Kepler,  by  casting  nativities,  eked  out  his 
miserable  pittance  as  royal  astronomer.  So  late  even 
as  the  reign  of  Charles  II.,  Lilly,  a  famous  astrolo- 
ger, was  called  before  a  committee  of  the  House  of 


14  THE   HISTORY. 

Commons,  to  give  his  opinion  on  the  probable  issue 
of  some  enterprise  then  under  consideration. 

However  foolish  the  system  of  Astrology  may 
have  been,  it  preserved  the  science  of  Astronomy 
during  the  Dark  Ages,  and  prompted  to  accurate 
observation  and  diligent  study  of  the  heavens. 

The  Copemican  System.^About  the  commence- 
ment of  the  sixteenth  century.  Copernicus,  breaking 
away  from  the  theory  of  Ptolemy,  that  was  still 
taught  in  the  institutions  of  learning  in  Europe, 
revived  the  theory  of  Pythagoras.  He  saw  how  beau- 
tifully simple  is  the  idea  of  considering  the  sun  the 
grand  center  about  which  revolve  the  earth  and  the 
planets.  He  noticed  how  constantly,  when  we  are 
riding  swiftly,  we  forget  our  own  motion,  and  think 
that  the  trees  and  fences  are  gliding  by  us  in  the 
contrary  direction.  He  applied  this  thought  to  the 
movements  of  the  heavenly  bodies,  and  maintained 
that,  instead  of  all  the  starry  host  revolving  about 
the  earth  once  in  twenty-four  hours,  the  earth  simply 
turns  on  its  own  axis,  and  thus  produces  the  ap- 
parent daily  revolution  of  the  sun  and  stars  ;  while 
the  yearly  motion  of  the  earth  about  the  sun,  trans- 
ferred in  the  same  manner,  would  account  for  the 
solar  movements. 

Though  Copernicus  thus  simplified  the  Ptolemaic 
theory,  he  yet  found  that  the  idea  of  circular  orbits 
for  the  planets  would  not  explain  all  the  phenomena, 
and  therefore  retained  the  "cycles  and  epicycles" 
Alfonso  had  so  heartily  condemned.  For  forty  years, 
this  illustrious  astronomer  carried  on  his  observa- 
tions in  the  upper  part  of  a  humble,  dilapidated 


Kepler's  laws.  15 

farm-house,  through  the  roof  of  which  he  had  an 
unobstructed  view  of  the  sky.  The  work  containing 
his  theory  was  published  just  in  time  to  be  laid  upon 
his  death-bed. 

Tycho  Brahe,  a  celebrated  Danish  astronomer, 
next  propounded  a  modification  of  the  Copernican 
system.  He  rejected  the  idea  of  cycles  and  epicycles, 
but,  influenced  by  certain  passages  of  Scripture, 
maintained,  with  Ptolemy,  that  the  earth  is  the 
center,  and  that  all  the  heavenly  bodies  daily  re- 
volve about  it  in  circular  orbits.  Brahe  was  a  noble- 
man of  wealth,  and,  in  addition,  received  large  sums 
of  money  from  the  government.  He  erected  a  mag- 
nificent observatory,  and  made  many  beautiful  and 
rare  instruments.  Clad  in  his  robes  of  state,  he 
watched  the  heavens  with  the  intelligence  of  a 
philosopher  and  the  splendor  of  a  king.  His  inde- 
fatigable industry  and  zeal  resulted  in  the  accumu- 
lation of  a  vast  fund  of  astronomical  knowledge, 
which,  however,  he  lacked  the  ability  to  apply  to  any 
further  advance  in  science. 

His  pupil,  Kepler,  saw  these  facts,  and  in  his  fruit- 
ful mind  they  germinated  into  three  great  truths, 
called  Kepler's  laws.  These  form  one  of  the  most 
precious  conquests  of  the  human  mind.  They  are 
the  three  arches  of  the  bridge  over  which  Astronomy 
crossed  the  gulf  between  the  Ptolemaic  and  Coper- 
nican systems. 

Kepler's  Laws. — Kepler,  taking  the  investigations 
of  his  master,  Tycho  Brahe,  determined  to  find  what 
is  the  exact  shape  of  the  orbits  of  the  planets.  He 
adopted  the  Copernican  theory — that  the  sun  is  the 


10  THE  HISTORY. 

center  of  the  system.  At  that  time,  all  believed  the 
orbits  to  be  circular.  They  reasoned  thus  :  the  circle 
is  perfect ;  it  is  the  most  beautiful  figure  in  nature  ; 
it  has  neither  beginning  nor  ending  ;  therefore,  it  is 
the  only  form  worthy  of  God,  and  He  must  have 
used  it  for  the  orbits  of  the  worlds  He  has  made. 

Imbued  with  this  romantic  view,  Kepler  com- 
menced with  a  rigorous  comparison  of  the  places  of 
the  planet  Mars  as  observed  by  Brahe,  with  the 
places  as  stated  by  the  best  tables  that  could  be  com- 
puted on  the  circular  theory.  For  a  time,  they 
agreed,  but  in  certain  portions  of  the  orbit  the  obser- 
vations of  Brahe  would  not  fit  the  computed  place 
by  eight  minutes  of  a  degree.  Believing  that  so 
good  an  astronomer  could  not  be  mistaken  as  to  the 
facts,  Kepler  exclaimed.  "  Out  of  these  eight  minutes 
we  will  construct  a  new  theory  that  will  explain  the 
movements  of  all  planets." 

He  resumed  his  work,  and  for  eight  years  con- 
tinued to  imagine  every  conceivable  hypothesis,  and 
then  patiently  to  test  it — "hunt  it  down,"  as  he 
called  it.  Each  in  turn  proved  false,  until  nineteen 
had  been  tried.  He  then  determined  to  abandon  the 
circle  and  to  adopt  another  form.  The  ellipse  sug- 
gested itself  to  his  mind.  Let  us  see  how  this  figure 
is  made. 

Attach  a  thread  to  two  pins,  as  at  FF  in  the  figure  ; 
next,  move  a  pencil  along  with  the  thread,  the  latter 
being  kept  tightly  stretched,  and  the  point  will  mark 
a  curve,  flattened  in  proportion  to  the  length  of  the 
string, — the  longer  the  string,  the  nearer  a  circle 
will  the  figure  become.      This  figure  is  the  ellipse. 


KEPLER  S   LAWS. 


17 


The  two  points  F  F  are  called  the  foci  (singular, 
focus).  We  can  now  understand  Kepler's  attempt, 
and  the  triumph  which  crowned  his  seventeen  years 
of  unflagging  toil. 


Fifl.  a. 


First  Law. — With  this  figure  he  constructed  an 
orbit  having  the  sun  at  the  center,  and  again  fol- 
lowed the  planet  Mars  in  its  course.  But  very  soon 
there  was  as  great  a  discrepancy  between  the  ob- 
served and  computed  places  as  before.  Undismayed 
by  this  failure,  Kepler  assumed  another  hypothesis, 
and  determined  to  place  the  sun  at  one  of  the  foci 
of  the  ellipse.  Once  more  he  "hunted  down"  the 
theory.  For  a  whole  year  he  traced  the  planet  along 
the  imaginary  orbit,  and  it  did  not  diverge.  The 
truth  was  disciovered  at  last,  and  Kepler  (1609)  an- 
nounced his  first  great  law — 

Planets  revolve  in  ellipses,  with  the  sun  at  one  focus. 


18  THE   HISTORY. 

Second  Law. — Kepler  knew  that  the  planets  do 
not  move  with  equal  velocity  in  the  different  parts 
of  their  orbits.  He  next  set  about  establishing  some 
law  by  which  this  speed  could  be  determined,  and 
the  place  of  the  planet  computed.  He  drew  an 
ellipse,  and  once  more  marked  the  various  positions 
of  the  planet  Mars.  He  soon  found  that  when  at 
its  perihelion  (point  nearest  the  sun)  its  motion 
is  fastest,  but  when  at  its  aphelion  (point  furthest 
from  the  sun)  its  motion  is  slowest.  Again  he 
"hunted  down"  various  hypotheses,  until,  at  last. 


he  discovered  that  though,  in  going  from  B  to  A,  the 
planet  moves  more  slowly,  and  from  D  to  C  more 
rapidly,  yet  the  space  inclosed  between  the  lines  SB 
and  SA  is  equal  to  that  inclosed  between  SD  and  SC. 
Hence  the  second  law — 

A  line  connecting  the  center  of  the  earth  with  the 
center  of  the  sun  passes  over  equal  spaces  in  equal 
times. 

Third  Law. — Kepler,  not  satisfied  with  the  dis- 
covery of  these  laws,  now  determined  to  ascertain  if 
there  were  not  some  relation  existing  between  the 


GALILEO.  19 

times  of  the  revolutions  of  the  planets  about  the  sun 
and  their  distances  from  that  body.  With  the  same 
wonderful  patience,  he  took  the  figures  of  Tycho 
Brahe,  and  began  to  compare  them.  He  tried  them 
in  every  imaginable  relation.  Next  he  took  their 
squares,  then  he  attempted  their  cubes.  Here  was 
the  secret ;  but  he  toiled  around  it,  made  a  blunder, 
and  waited  for  months,  until,  once  more,  his  patience 
triumphed,  and  he  reached  (1618)  the  third  law — 

The  squares  of  the  times  of  revolution  of  the  planets 
about  the  sun  are  proportional  to  the  cubes  of  their 
mean  distances  from  the  sun.* 

In  rapture  over  the  discovery  of  these  three  laws, 
so  marked  by  that  Divine  simplicity  which  pervades 
all  the  laws  of  nature,  Kepler  exclaimed,  ''Nothing 
holds  me.  The  die  is  cast.  The  book  is  written,  to 
be  read  now  or  by  posterity,  I  care  not  which.  It 
may  well  wait  a  century  for  a  reader,  since  God  has 
waited  six  thousand  years  for  an  observer,  f 

Galileo. — Contemporary  with  Kepler  was  the  great 
Florentine  philosopher,  Galileo.  He  discovered  the 
laws  of  the  pendulum  and  of  falling  bodies,  as  we 
have  already  learned  in  Physics.  He  was,  however, 
educated  in  and  believed  the  Ptolemaic  system.  A 
disciple  of  the  Copernican  theory  happening  to  come 
to  Pisa,  where  Galileo  was  teaching  as  professor  in 

*  For  example  :  Tlie  square  of  Jupiter's  period  is  to  the  square  of  Mars's  period,  as 
the  cube  of  Jupiter's  distance  is  to  the  cube  of  Mars's  distance  ;  or,  representing  the 
earth's  time  of  revolution  by  P,  and  her  distance  from  the  sun  by  p,  then  letting  D  and 
d  represent  the  same  in  another  planet,  we  have  the  proportion  P»  :  D'  :  :  p'  :  d'. 

t  Kepler,  strangely  enough,  believed  in  the  "  Music  of  the  Spheres."  He  made 
Saturn  and  Jupiter  take  the  bass,  Mars  the  tenor,  Earth  and  Venus  the  counter,  and 
Mercury  the  treble.  This  shows  what  a  streak  of  folly  or  superstition  may  run  through 
the  cliaracter  of  the  noblest  man.  However,  as  Johnson  says,  a  mass  of  metal  may  be 
gold,  though  there  be  in  it  &  littlo  vein  of  tiu. 


20  THE  HISTORY. 

the  University,  drew  his  attention  to  its  simplicity 
and  beauty.  His  clear,  discriminating  mind  per- 
ceived its  perfection,  and  he  henceforth  advocated  it 
with  all  the  ardor  of  his  unconquerable  zeal.  Soon 
after,  he  learned  that  one  Jansen,  a  Dutch  watch- 
maker, had  invented  a  contrivance  for  making  dis- 
tant objects  appear  near.  With  his  profound  knowl- 
edge of  optics  and  philosophical  instruments,  Galileo 
caught  the  idea,  and  soon  had  a  telescope  completed. 
It  was  a  very  simple  affair — only  a  piece  of  lead  pipe 
with  a  lens  set  at  each  end  ;  but  it  was  destined  to 
overthrow  the  old  Ptolemaic  theory,  and  revolu- 
tionize the  science  of  Astronomy. 

Discoveries  made  with  the  Telescope. — Galileo 
now  examined  the  moon.  He  saw  her  mountains  and 
valleys,  and  watched  the  dense  shadows  upon  her 
plains.  On  January  8,  1610,  he  turned  the  telescope 
toward  Jupiter.  Near  it  he  saw  three  bright  stars, 
as  he  considered  them,  which  were  invisible  to  the 
naked  eye.  The  next  night  he  noticed  that  they  had 
changed  their  relative  positions.  Astonished  and 
perplexed,  he  waited  three  days  for  a  fair  night  in 
which  to  resume  his  observations.  The  fourth  night 
was  favorable,  and  he  found  the  three  stars  had 
again  shifted.  Night  after  night  he  watched  them, 
discovered  a  fourth  star,  and  finally  found  that  they 
were  rapidly  revolving  around  Jupiter,  each  in  its 
elliptical  orbit,  with  its  own  rate  of  motion,  and  all 
accompanying  the  planet  in  its  journey  around  the 
sun.  Here  was  a  miniature  Copernican  system,  hung 
up  in  the  sky  for  every  one  to  see  and  examine  for 
liimself. 


NEWTON.  21 

Reception  of  the  Discoveries. — Galileo  met  with 
the  most  bitter  opposition.  Many  refused  to  look 
through  the  telescope  lest  they  might  become  victims 
of  the  philosopher's  magic.  Some  prated  of  the 
wickedness  of  digging  out  valleys  in  the  fair  face  of 
the  moon.  Others  doggedly  clung  to  the  theory  they 
had  held  from  their  youth.*  But  the  truth  of  the 
Copernican  system  was  now  fully  established.  Phil- 
osophers gradually  adopted  this  view,  and  the  Ptole- 
maic theory  became  a  relic  of  the  past. 

Newton,  a  young  man  of  twenty-four  years,  was 
spending  the  summer  of  1666  in  the  country,  on 
account  of  the  plague  which  prevailed  at  Cambridge, 
his  place  of  residence.  One  day,  while  sitting  in  a 
garden,  an  apple  chanced  to  fall  to  the  ground  near 
him.  Reflecting  upon  the  strange  power  that  causes 
all  bodies  thus  to  descend  to  the  earth,  and  remem- 
bering that  this  force  continues,  even  when  we  as- 
cend to  the  tops  of  high  mountains,  the  thought  oc- 
curred to  his  mind,  "May  not  this  same  force  extend 
to  a  great  distance  out  in  space  ?  Does  it  not  reach 
the  moon  ?  " 

Laws  of  Motion. — To  understand  the  reasoning 
that  now  occupied  the  mind  of  Newton,  let  us  apply 
the  laws  of  motion  as  we  have  learned  them  in 

*  As  a  specimen  of  the  arguments  adduced  against  the  new  system,  the  following  l>y 
Sizzi  is  a  fair  instance.  "  There  are  seven  windows  in  the  head,  through  which  the  air 
is  admitted  to  the  body,  to  enlighten,  to  warm,  and  to  nourish  it,— two  nostrils,  two 
eyes,  two  ears,  and  one  mouth.  So  in  the  heavens  there  are  two  favorable  stars,  Jupiter 
and  Venus ;  two  unpropitious.  Mars  and  Saturn  ;  two  luminaries,  the  Sun  and  Moon  ; 
and  Mercury  alone,  undecided  and  indifferent.  From  which,  and  from  many  other  phe- 
nomena in  Nature,  such  as  the  seven  metals,  etc.,  we  gather  that  the  number  of  planets 
is  necessarily  seven.  Moreover,  the  satellites  are  invisible  to  the  naked  eye,  can  exercise 
no  influence  over  the  earth,  and  would  be  useless,  and  therefore  do  not  exist,  liesides, 
the  week  is  divided  into  seven  days,  which  are  named  from  the  seven  planets.  Now,  if 
we  increase  the  number  of  planets,  this  whole  system  falls  to  the  ground." 


22  THE   HISTORY. 

Physics.  "When  a  body  is  set  in  motion,  it  will  con- 
tinue to  move  forever  in  a  straight  line,  unless 
another  force  is  applied.  As  there  is  no  friction  in 
space,  the  planets  do  not  lose  any  of  their  original 
velocity,  but  move  now  with  the  same  speed  which 
they  received  at  the  beginning.  But  this  would 
make  them  all  pass  along  straight  lines,  and  not  cir- 
cular orbits.  What  causes  the  curve  ?  Obviously, 
another  force.  For  example :  I  throw  a  stone  into 
the  air.  It  does  not  move  in  a  straight  line,  but  in 
a  curve,  because  the  earth  constantly  bends  it  down- 
ward. 

Application. — Just  so  the  moon  is  moving  around 
the  earth,  not  in  a  straight  line,  but  in  a  curve.  Can 
it  not  be  that  the  earth  bends  it  downward,  just  as 
it  does  the  stone  ?  Newton  knew  that  a  stone  falls 
toward  the  earth  sixteen  feet  the  first  second.  He 
conceived,  after  a  careful  study  of  Kepler's  laws, 
that  the  attraction  of  the  earth  diminishes  according 
to  the  square  of  the  distance.  He  supposed  (accord- 
ing to  the  measurement  then  received)  that  a  body 
on  the  surface  of  the  earth  is  exactly  four  thousand 
miles  from  the  center.  He  now  applied  this  imag- 
inary law.  Suppose  the  body  is  removed  four  thou- 
sand miles  from  the  surface  of  the  earth,  or  eight 
thousand  miles  from  the  center.  Then,  as  it  is  twice 
as  far  from  the  center,  its  weight  will  be  diminished 
2^,  or  4  times.  If  it  were  placed  3,  4,  5,  10  times  fur- 
ther away,  its  weight  would  then  decrease  9,  16,  25, 
100  times.  If,  then,  the  stone  at  the  surface  of  the 
earth  (four  thousand  miles  from  the  center)  falls 
sixteen  feet  the  first  second,  at  eight  thousand  miles 


NEWTON.  23 

it  would  fall  only  four  feet ;  at  240,000  miles,  or  the 
distance  of  the  moon,  it  would  fall  only  about  one.- 
twentieth  of  an  inch  (exactly  .053). 

Next  the  question  arose,  "How  far  does  the  moon 
fall  toward  the  earth,  i.  e.,  bend  from  a  straight  line, 
every  second  ?  "  For  sixteen  years,  with  a  patience 
rivaling  Kepler's,  this  philosopher  sought  to  solve  the 
problem.  He  toiled  over  interminable  columns  of 
figures,  to  find  how  much  the  moon's  path  curves 
each  second.  At  last,  he  reached  a  result,  which  was 
nearly,  but  not  quite,  exact.  Disappointed,  he  laid 
aside  his  calculations.  Repeatedly  he  reviewed 
them,  but  could  not  find  a  mistake.  At  length, 
while  in  London,  he  learned  of  a  new  and  more 
accurate  measurement  of  the  distance  from  the  cir- 
cumference to  the  center  of  the  earth.  He  hastened 
home,  inserted  this  new  value  in  his  calculations, 
and  soon  found  that  the  result  would  be  correct. 
Overpowered  by  the  thought  of  the  grand  truth  just 
before  him,  his  hand  faltered,  and  he  called  upon  a 
friend  to  complete  the  computation. 

From  the  moon,  Newton  passed  on  to  the  other 
heavenly  bodies,  calculating  and  testing  their  orbits. 
Finally,  he  turned  his  attention  to  the  sun,  and,  by 
reasoning  equally  conclusive,  proved  that  the  attrac- 
tion of  that  great  central  orb  compels  all  the  planets 
to  revolve  about  it  in  elliptical  orbits,  and  holds  them 
with  an  irresistible  power  in  their  appointed  paths.* 

*  "  Do  not  understand  me  at  all  as  saying  there  is  no  mystery  about  the  planets'  mo- 
tion. There  is  just  one  single  mystery,— gravitation  ;  and  it  is  a  very  profound  one. 
How  it  is  that  an  atom  of  matter  can  attract  another  atom,  no  matter  how  great  the 
distance,  no  matter  what  intervening  substance  there  may  be  ;  how  it  will  act  upon 
it,  or  at  least  behave  as  if  it  acted  upon  it,— I  do  not  know,  I  cannot  tell.  Whether  they 


24  THE  HISTORY. 

At  last,  he  announced  this  grand  Law  of  Gravita- 
tion :  Every  particle  of  matter  in  the  universe  at- 
tracts every  other  particle  of  matter  with  a  force 
directly  proportional  to  its  quantity  of  matter,  and 
decreasing  as  the  square  of  the  distance  increases. 


_^  II.    SPACE. 

We  now  in  imagination  pass  into  space,  which- 
stretches  out  in  every  direction,  without  bounds  or 
measures.  We  look  up  to  the  heavens,  and  try  to 
locate  some  object  among  the  mazes  of  the  stars. 
Bewildered,  we  feel  the  necessity  of  some  system  of 
measurement.  Let  us  try  to  understand  the  one 
adopted  by  astronomers. 

The  Celestial  Sphere. — The  blue  arch  of  the  sky, 
as  it  appears  to  be  spread  over  us,  is  termed  the 
Celestial  Sphere.  There  are  two  points  to  be  noticed 
here. 

First,  that  so  far  distant  is  this  imaginary  arch 
from  us,  that  if  any  two  parallel  lines  from  different 
parts  of  the  earth  were  drawn  to  this  Sphere,  they 
would  apparently  intersect.  Of  course,  this  could 
not  be  the  fact ;  but  the  distance  is  so  immense,  that 
we  are  unable  to  distinguish  the  little  difference  of 

are  puslied  together  l)y  means  of  an  intervening  ether,  or  what  is  the  action,  I  cannot 
und(T.stand.  It  stands  with  me  along  with  the  fact,  that,  when  I  will  my  arm  to  rise,  it 
rises.  It  is  inscrutable.  All  the  explanations  that  have  been  given  of  it  seem  to  me 
merely  to  darken  counsel  with  words  and  no  understanding.  They  do  not  remove  the 
difficulty  at  all.  If  I  were  to  say  what  I  really  believe,  it  would  be,  that  the  motion  of 
the  spheres  of  the  material  universe  stand  in  some  such  relation  to  Him  in  whom  all 
things  exist,  the  ever-present  and  omnipotent  God,  as  the  motions  of  my  body  do  to  my 
will :  I  do  not  know  bow,  and  never  expect  to  know."— Pro/.  Young. 


SPACE.  25 

four  or  even  eight  thousand  miles,  and  the  two  lines 
would  seem  to  unite  :  so  we  must  consider  this  great 
earth  as  a  mere  speck  or  point  at  the  center  of  the 
Celestial  Sphere. 

Second,  that  we  must  neglect  the  entire  diameter 
of  the  earth's  orbit,  so  that  if  we  should  draw  two 
parallel  lines,  one  from  each  end  of  the  earth's  orbit, 
to  the  Celestial  Sphere,  although  these  lines  would 
be  nearly  186,000,000  miles  apart,  yet  they  would 
appear  to  pierce  the  Sphere  at  the  same  point ;  which 
is  to  say,  that,  at  that  enormous  distance,  186,000,000 
miles  shrink  to  a  point.  Consequently,  in  all  parts 
of  the  earth,  and  in  every  part  of  the  earth's  orbit, 
we  see  the  fixed  stars  in  the  same  place. 

This  sphere  of  stars  surrounds  the  earth  on  every 
side.  In  the  daytime,  we  cannot  see  the  stars  be- 
cause of  the  superior  light  of  the  sun  ;  but,  with  a 
telescope,  they  can  be  traced,  and  an  astronomer  will 
find  certain  stars  as  well  at  noon  as  at  midnight. 

One  half  of  the  sphere  is  constantly  visible  to  us  ; 
and  so  far  distant  are  the  stars,  that  we  see  just  as 
much  of  the  sphere  as  we  should  if  the  upper  part  of 
the  earth  were  removed,  and  we  were  to  stand  four 
thousand  miles  further  away,  or  at  the  center  of  the 
earth,  where  our  view  would  be  bounded  by  a  great 
circle  of  the  earth. 

On  the  concave  surface  of  the  Celestial  Sphere, 
there  are  imagined  to  be  drawn  three  systems  of 
circles :  the  Horizon,  the  Equinoctial,  and  the 
Ecliptic  System.  Each  of  these  has  (1)  its  Prin- 
cipal Circle,  (2)  its  Suhordinate  Circles,  (3)  its 
Points,  and  (4)  its  Measurements. 


26 


SPACE. 


Fig.  7. 

z 


1.    THE    HORIZON    SYSTEM. 

(a)  The  Principal  Circle  is  the  Rational  Horizon. 
This  is  the  great  circle  whose  plane,  passing  through 
the  center  of  the  earth,  separates  the  visible  from  the 
invisible  heavens.  The  Sensible  Horizon  is  the  small 
circle  where  the  earth  and  the  sky  seem  to  meet :  it 
is  parallel  to  the  rational  horizon,  but  distant  from 
it  the  semi-diameter  of  the  earth.  No  two  places 
have  the  same  sensible  horizon  :  any  two,  on  opposite 
sides  of  the  earth,  have  the  same  rational  horizon. 

(6)  The  Subordinate  Cir- 
cles are  the  Prime  Verti- 
cal circle,  and  the  Merid- 
ian. A  vertical  circle  is 
one  passing  through  the 
poles  of.  the  horizon  (ze- 
nith, and  nadir).  The 
Prime  Vertical  is  a  verti- 
cal circle  passing  througl 
the  East  and  West  points 
The  Meridian  is  a  vertical 
circle  passing  through  the 

E,  center  of  earth  ;  Z,  zenith  ;  Z',  nadir ;  ,  .  . 

PP',  axis  of  earth;   HAH',    horizon;  S,  a   Nortll   aud  SOUtll  pOlUtS. 
star;   ZSZ',  rertical  circle  passing  through 

a  ■  AS,  altitude  of  star  ;  ZS,  zenith  distance         (q)    The     PolUtS    are    the 
of  star  ;  B.' A.,  azimuth  of  star.  ^ 

Zenith,  the  Nadir,  and  the 
K,  S.,  E.,  and  W.  points.  The  Zenith  is  the  point 
directly  overhead,  and  the  Nadir,  the  one  directly 
underfoot.  They  are  also  the  poles  of  the  horizon — 
i.  e.,  the  points  where  the  axis  of  the  horizon  pierces 
the  Celestial  Sphere.  The  N.,  S.,  E.,  and  W.  points 
are  familiar. 


z' 


THE  HORIZON  SYSTEM.  2? 

(d)  The  Measurements  are  Azimuth,  Amplitude, 
Altitude,  and  Zenith  distance. 

Azimuth  is  the  distance  from  the  meridian,  meas- 
ured east  or  west,  on  the  horizon,  to  a  vertical  circle 
passing  through  the  object. 

Amplitude  (the  complement  of  Azimuth)  is  the 
distance  from  the  Prime  Vertical,  measured  on  the 
horizon,  north  or  south. 

Altitude  is  the  distance  from  the  horizon,  meas- 
ured on  a  vertical  circle,  toward  the  zenith. 

Zenith  Distance  (the  complement  of  Altitude)  is 
the  distance  from  the  zenith,  measured  on  a  vertical 
circle,  toward  the  horizon. 

The  Horizon  system  is  one  commonly  used  in 
observations  with  Mural  Circles,  and  Transit  Instru- 
ments. 

2.   THE    EQUINOCTIAL    SYSTEM. 

(a)  The  Principal  Circle  is  the  Equinoctial.  This 
is  the  Celestial  Equator^  or  the  earth's  equator  ex- 
tended to  the  Celestial  Sphere.  At  all  places  between 
the  equator  and  the  pole,  the  celestial  equator  is  in- 
clined to  the  horizon  at  an  angle  equal  to  the  dis- 
tance of  the  zenith  of  the  place  from  the  pole.  * 

{h)  The  Subordinate  Circles  are  the  Hour  Circles 
(Right  Ascension  Meridians),  the  Colures,  and  the 
Declination  Parallels. 

*  Tlie  latitude  of  a  place  is  its  distance  from  the  equator,  and  this  equals  the  distance 
of  the  zenith  of  the  place  from  the  equinoctial.  Hence,  having  given  the  latitude  of  a 
place,  to  find  the  height  of  the  celestial  equator  above  its  horizon,  subtract  the  latitude 
from  90°,  and  the  remainder  is  the  required  angular  distance.  In  like  manner,  the  lati- 
tude subtracted  from  90°  gives  the  co-latitude  of  the  place— the  complement  of  the 
latitude. 


28  SPACE. 

The  Hour  Circles  are  thus  located.  The  Equi- 
noctial is  divided  into  360°,  equal  to  twenty-four 
hours  of  motion — thus  making  15  equal  to  one  hour 
of  motion.  Through  these  divisions  run  twenty-four 
meridians,  each  constituting  an  hour  of  motion 
(time)  or  15^  of  space.  The  Hour  Circles  may  be 
conceived  as  meridians  of  terrestrial  longitude  (15" 
apart)  extended  to  the  Celestial  Sphere. 

The  Colures  are  two  principal  meridians ;  the 
Equinoctial  Cohire  is  the  meridian  passing  through 
the  equinoxes  ;  the  Solstitial  Colure  is  the  meridian 
passing  through  the  solstitial  points. 

The  Declination  Parallels  are  small  circles 
parallel  to  the  Equinoctial ;  or  they  may  be  conceived 
as  the  parallels  of  terrestrial  latitude  extended  to  the 
Celestial  Sphere. 

(c)  The  Points  are  the  Celestial  Poles,  and  the 
Equinoxes. 

The  Celestial  Poles  are  the  points  where  the 
axis  of  the  earth  extended  pierces  the  Celestial 
Sphere,  and  are  the  extremities  of  the  celestial  axis, 
as  the  poles  of  the  earth  are  the  extremities  of  the 
earth's  axis.  The  North  Pole  is  marked  very  nearly 
by  the  Xorth  Star,  and  every  direction  from  that  is 
reckoned  south,  and  every  direction  toward  that  is 
reckoned  north,  however  it  may  conflict  with  our 
ideas  of  the  points  of  the  compass. 

The  Equinoxes  are  the  points  where  the  Equinoc- 
tial and  the  Ecliptic  (the  sun's  apparent  path  through 
the  heavens)  intersect. 

(c7)  The  Measurements  are  Right  Ascension  (R.  A.), 
Declination,  and  Polar  Distance, 


THE  EQUINOCTIAL  SYSTEM.  S9 

Right  Ascension  is  distance  from  the  Vernal 
Equinox,  measured  on  the  equinoctial  eastward 
to  the  meridian  which  passes  through  the  body. 
R.  A.  corresponds  to  terrestrial  longitude,  and  may 
extend  to  360°  East,  instead  of  180''  as  on  the  earth. 
R.  A.  is  never  measured  westward.  The  starting 
point  is  the  meridian  passing  through  the  vernal 
equinox,  as  the  meridian  passing  through  Green- 
wich is  the  point  from  which  terrestrial  longitude  is 
measured. 

Declination  is  distance  from  the  equinoctial, 
measured  on  any  Hour  Circle  or  meridian  north  or 
south.     It  corresponds  to  terrestrial  latitude. 

Polar  Distance  (the  complement  of  Declination) 
is  the  distance  from  either  Pole,  measured  on  an 
Hour  Circle. 

The  Equinoctial  System  is  largely  used  by  modern 
astronomers,  and  accompanies  the  Equatorial  Tele- 
scope, Sidereal  Clock,  and  Chronographs  of  the  best 
Observatories. 

3.    THE    ECLIPTIC    SYSTEM. 

(a)  The  Principal  Circle  is  the  Ecliptic.  This  is 
the  apparent  path  of  the  sun  in  the  heavens.  It  is 
inclined  to  the  equinoctial  23^°  (23°  27'  15",  Jan.  1, 
1884),  which  measures  the  inclination  of  the  Earth's 
Equator  to  its  orbit,  and  is  called  the  obliquity  of  the 
ecliptic.     (See  p.  58.) 

The  inclination  of  the  ecliptic  to  the  horizon,  unlike 
that  of  the  equinoctial,  varies  at  different  times  of 
the  year.  The  angle  that  the  ecliptic  makes  with 
the  horizon  is  greatest  when  the  vernal  equinox  is 


30  SPACE. 

on  the  western  horizon  and  the  autumnal  on  the 
eastern ;  it  is  least  when  the  vernal  equinox  is  on 
the  eastern  horizon  and  the  autumnal  on  the  western.* 

{b)  The  Subordinate  Circles  are  Circles  of  Celestial 
Longitude,  and  Parallels  of  Celestial  Latitude. 

The  Circles  of  Celestial,  Longitude  are  now 
seldom  employed.  They  are  measured  on  the  Eclip- 
tic, as  circles  of  Right  Ascension  (R.  A.)  are  meas- 
ured on  the  Equinoctial. 

The  Parallels  of  Celestial  Latitude  are  little 
used.  They  are  small  circles  drawn  parallel  to  the 
ecliptic,  as  parallels  of  declination  are  drawn  parallel 
to  the  equinoctial. 

(c)  The  Points  are  the  Foles  of  the  Ecliptic,  the 
Equinoxes,  and  the  Solstices. 

The  Poles  of  the  Ecliptic  are  the  points  where  the 
axis  of  the  earth's  orbit  meets  the  Celestial  Sphere. 

The  Equinoxes  are  the  points  where  the  ecliptic 
intersects  the  equinoctial.  The  place  where  the  sun 
crosses  the  equinoctial  f  in  going  north,  which  occurs 
about  the  21st  of  March,  is  called  the  Vernal  Equinox. 
The  place  where  the  sun  crosses  the  equinoctial  in 
going  south,  which  occurs  about  the  21st  of  Septem- 
ber, is  called  the  Autumnal  Equinox.  The  Solstices 
are  the  two  points  of  the  ecliptic  most  distant  from 
the  Equator  ;  or  they  may  be  considered  to  mark  the 
sun's  furthest  declination  north  and  south  of  the 
equinoctial.     The  Summer  Solstice  occurs  about  the 

*  In  the  former  instance,  the  angle  is  eqnal  to  the  co-latitude,  plus  23J°  (the  inclina- 
tion of  the  ecliptic  to  the  equinoctial) ;  and,  in  the  latter,  the  co-latitude  minus  2;U°. 
Thus,  at  the  latitude  of  New  York,  it  varies  from  90°  —  41*  +  ■2:i\  °  =  72J  "  ;  to  90°  — 
41°  —  231*  =  25J'.  In  the  one  case,  the  summer  solstice  ia  on  the  meridian  of  the 
place,  and,  in  the  other,  the  winter. 

t  "  This  is  commonly  called  '  crossing  the  line.' ' 


THE  ECLIPTIC  SYSTEM.  81 

31st  of  June ;  the  Winter  Solstice  occurs  about  the 
21st  of  December. 

(d)  The  Measurements  are  Celestial  Longitude  and 
Latitude. 

Celestial  Longitude  is  distance  from  the  Vernal 
Equinox  measured  on  the  ecliptic,  eastward. 

Celestial  Latitude  is  distance  from  the  ecliptic 
measured  on  a  Subordinate  Circle,  north  or  south. 

THE  ZODIAC. 
A  belt  of  the  Celestial  Sphere,  8°  on  each  side  of 
the  ecliptic,  is  styled  the  Zodiac.  This  is  of  very- 
high  antiquity,  having  been  in  use  among  the 
ancient  Hindoos  and  Egyptians.  The  Zodiac  is 
divided  into  twelve  equal  parts — of  30°  each — called 
Signs,  to  each  of  which  a  fanciful  name  is  given. 
The  following  are  the  names  of  the 

SIGNS    OF    THE    ZODIAC. 


Libra a 

Scorpio Ttj^ 

Sagittarius f 

Capricornus . .  V5> 

Aquarius oji 

Pisoes X 


Aries HP 

Taurus y 

Gemini n 

Cancer 25 

Leo ^ 

Virgo nj 

"  The  first,  nn,  indicates  the  horns  of  the  Ram  ;  the 
second,  «  ,  the  head  and  horns  of  the  Bull ;  the  barb 
attached  to  a  sort  of  letter,  "1 ,  designates  the  Scor- 
pion ;  the  arrow,  i  ,  sufficiently  points  to  Sagitta- 
rius ;  V?  is  formed  from  the  Greek  letters,  rp,  the  two 
first  letters  of  rpdyog,  a  goat.  Finally,  a  balance, 
the  flowing  of  water,  and  two  fishes,  tied  by  a  string, 
may  be  imagined  in  ^,  ^,  and  X,  the  signs  of  Libra, 
Aquarius,  and  Pisces."    (See  pp.  210,  295.) 


32  PRACTICAL   QUESTIONS. 


PRACTICAL    QUESTIONS. 

1.  How  high  is  the  Xorth  Star  above  your  horizon  ? 

2.  What  is  the  sun's  right  ascension  at  the  autumnal  equinox  ?  At  the 
vernal  equinox  ? 

3.  What  was  the  first  discovery  made  by  the  telescojie  ? 

4.  How  high  above  the  horizon  of  any  place  are  the  equinoctial  points 
when  they  pass  the  meridian  ? 

5.  Jupiter  revolves  around  the  sun  in  12  of  our  years.  Assuming  the 
earth's  distance  from  the  sun  to  be  93,000,000  miles,  compute  Jupiter's  dis- 
tance by  applying  Kepler's  third  law. 

6.  The  latitude  of  Albany  is  42°  39'  X  ;  what  is  the  sun's  meridian 
altitude  at  that  place  when  it  is  in  the  celestial  equator  ? 

7.  What  is  the  co-latitude  of  a  place  ? 

8.  What  is  the  declination  of  the  zenith  of  the  place  in  which  you 
reside? 

9.  Why  are  the  stars  generally  invisible  by  day  ? 

10.  Why  is  the  ecliptic  so  called  ? 

11.  Who  fii-st  taught  that  the  earth  is  round  ? 

12.  What  is  Astrology  ? 

13.  How  can  we  distinguish  the  fixed  stars  from  the  planets  ? 

14.  How  long  was  the  Ptolemaic  System  accepted  ? 

15.  In  what  respect  did  the  Copernican  System  differ  from  the  one  now 
received  ? 

16.  For  what  is  Astronomy  indebted  to  Galileo  ?     To  Newton  ? 

1 7.  What  is  the  amount  of  the  obliquity  of  the  ecliptic  1 

18.  Define  Zenith.  Xadir.  Azimuth.  Altitude.  Equinoctial.  Right 
Ascension.  Declination.  Equinox.  Ecliptic.  Colure.  Solstice.  Polar 
distance.     Zenith  distance.     The  Zodiac. 

19.  If  the  R.  A.  of  the  sun  be  80",  state  in  what  sign  he  is  then  located  ? 
160°  ?     280°  ? 

20.  ^^^ly  does  the  angle  which  the  ecliptic  makes  with  the  horizon  vary  ? 

21.  "\Miy  is  the  angle  which  the  celestial  equator  makes  with  the  horizon 
constant  ? 


II. 

THE   SOLAR   SYSTEM. 


'  In  them  hath  He  set  a  tabernacle  for  the  sun, 

"  This  world  was  once  a  Jluid  haze  of  light, 
Till  toward  the  center  set  the  starry  tides 
And  eddied  into  sttns,  that  wheeling  cast 
The  planets." — Tennyson. 


L  The  Sun  , 


1.  Distance. 

2.  Ljght  (t  Heat. 

3.  Apparent  Size, 

4.  Real    Dimen- 


sions. 


5.  Solar  Spots.. 


xn 

CD 

<! 
iJ 
O 
CO 

w 


IL  The  Planets. 


'  a.  Discovery. 
&.  Number  and  Location. 

c   Size. 

d.  Constituents. 

e.  Motion  across  Disk. 
/.  Cliange  iu  Kate. 
g.  Prove  the  Rotation  of  Sun. 
h.  Synodic  and  Sidereal  Rotation 
i.  Path  of  Spot-s. 
,;.  Individual  Motion. 
k.  Change  in  Form. 
I.  Perio<licity  of  Spots. 
m.  Planetary  Influence. 
TO.  Influence  on  Terrestrial  Heat,  etc 
0.  Heat  of  Spots. 
p.  Depression  of  Spots. 
q.   Brightness  of  Spots. 
r.  Faculw,  rice-grains,  etc. 
a.  Wilson's  Theory. 
h.  Present  Theory  (Kirchhoff's). 

How  Solar  Heat  is  Produced. 

a.  Common  Characteristics. 
6.  Comparison  of  Planets. 

c.  Properties  of  the  Ellipse. 

d.  Planetary  Orbits. 

e.  Comparative  Size  of  Planets. 
/.  Conjunction  of. 
g.  Are  Planets  Inhabited  ? 

t  A  top.  Division  of  Planets,  etc 

1.   VCLCAS. 

a.  Description. 
h.  Motion  in  Space. 

c.  Distance  from  Earth. 

d.  Dimensions. 

e.  Seasons. 
,  /.  Telescopic  Features. 

Vesds Repeat  same  Analysis  as  of  Mercury. 

/'a.  Dimensions. 
Rotundity. 
Apparent  &  Real  Motion. 

f  1.  Diumal  Mo- 
tion of  Sun. 

2.  Unequal  rate 
of  Motion. 

3.  Orbits     of 
Stars. 

4.  Unequal  Ve- 
locitiesofStars. 

5.  Appearance 
I  of  Stars,  etc. 
(  1.  Change     in 

appearance  of 
heavens. 


Physical  Con- 
stitition  . .  .  . 


-iNTRODrCTION.  . .   ■ 


2.  Mercurt ■ 


4.  The  Eakth  . 


—The  Moon.  -< 


—Eclipses. 


—The  Tides. 


a.  Motion. 
h.  Dimens'ns. 

c.  Librations. 

d.  L'g-t&Ht. 

e.  Cen.ofGrav. 
/.  Atmosph're. 
g.  Lunarians. 
h.  Earth-shine, 
i.  Phases. 
j.  Harv'stM'n. 
k.  Wet  Moon. 
I.   Nodes. 
TO.  Occulat'n. 
n.  Seasons. 
0.  Telescopic 

Features. 


2.  Yearly  path 
of  Sun. 

3.MovesN.&S. 
4.  Change  of 
Seasons,  etc.  20 
points  under 
l^  this  topic. 

/.  Precession  of  Equinoxes. 

g.  Nutation. 

U.  Refraction  &  Aberration. 

i.  Parallax. 

5.  Mars Same  Analysis  as  Mercury. 

6.  The  Minor  Planets. 

7.  Jupiter Same  Analysis  as  Mercury. 

8.  Saturn "  " 

9.  Uranus "  •' 

^  10.  Neptune "  " 

III.  Meteors,  and  Shooting  Stars.  )  Tlie  subjects  of  the  paragraphs  may  be  inserted 

IV.  Comets r     by  the  pupil,  to  complete  these  analyses,  at 

V.  The  Zodiacal  Light )     the  pleasure  of  the  teacher. 


Dium'l  Mo- 
tion of 
Earth. 


Yearly  Mo- 
tion    of 
Sun  :  its 
C  o  n  s  e  - 
queiices. 


THE    SOLAR    SYSTEM. 


INTRODUCTION. 

rr^HE  Solar  System  is  mainly  comprised  within 
~L     the  limits  of  the  Zodiac.     It  consists  of — 

1.  The  Sun — the  center. 

2.  The   major    planets — Vulcan  (undetermined),    Mercury,  Venus, 

Earth,  Mars,  Jupiter,  Saturn,  Uranus,  Neptune. 

3.  The  minor  planets,  at  present  (1884)  two  hundred  and  thirty- 

seven  in  number. 

4.  The  satellites,  or  moons,  twenty  in  number,  which  revolve  around 

the  different  planets. 

5.  Meteors  and  shooting-stars. 

6.  Thirteen  comets,  which  have  now  been  found,  by  a  second  re- 

turn,   to  move,    like   the   planets,    in  elliptic  paths,    and  to 
revisit  the   sun   periodically. 

7.  The  Zodiacal  Light. 

How  we  are  to  imagine  the  solar  system  to  our- 
selves.— We  are  to  think  of  it  as  suspended  in  space  ; 
being  held  up,  not  by  any  visible  object,  but  in 
accordance  with  the  law  of  Universal  Gravitation 
discovered  by  Newton,  whereby  each  planet  attracts 
every  other  planet  and  is  in  turn  attracted  by  all. 

First,  the  Sun,  a  great  central  globe,  so  vast  as  to 
overcome  the  attraction  of  all  the  planets,  and  com- 
pel them  to  circle  around  him;    next,  the  planets, 


36  THE  SOLAR  SYSTEM. 

each  turning  on  its  axis  while  it  flies  around  the  sun 

in  an  elliptical  orbit  :  then,  accompanying  these,  the 
satellites,  each  revolving  about  its  own  planet,  while 
all  whirl  in  a  dizzy  waltz  about  the  central  orb ; 
next,  the  comets,  rushing  across  the  planetary 
orbits  at  irregular  intervals  of  time  and  space  ;  and 
finally,  shooting-stars  and  meteors  darting  hither 
and  thither,  interweaving  all  in  apparently  inex- 
tricable confusion. 

To  make  the  picture  more  wonderful  still,  every 
member  is  flying  with  an  inconceivable  velocity,  and 
yet  with  such  accuracy  that  the  solar  system  is  the 
most  perfect  timepiece  known. 


I.     THE    SUN. 

Sign,   ©,  a  buckler  with  its  boss. 

Distance. — The  sun's  average  distance  from  the 
earth  is  nearly  93,000,000  miles.*  Since  the  earth's 
orbit  is  elliptical,  and  the  sun  is  situated  at  one  of  its 
foci,  the  earth  is  3,000,000  miles  further  from  the  sun 
in  aphelion  than  in  perihelion. 

*  The  sun's  distance  from  the  earth  is  determined,  as  we  shall  learn  hereafter  (see 
Celestial  Measurements),  by  means  of  the  solar  parallax.  In  the  former  editions  of  this 
work,  the  parallax  of  S".94 — deduced  principally  ftr)m  observations  upon  the  planet  Mars 
in  18'32— was  accepted.  This  gave  a  solar  distance  of  about  91|  million  miles,  and  has 
been  in  general  use  among  astronomers  until  recently.  The  obsen-ations  of  the  last  few 
years  have,  however,  shown  that  the  true  parallax  is  smaller,  and  that  the  sun  is  a 
little  further  off  than  was  supposed.  Astronomers  are  not  fully  agreed  upon  the  exact 
Xtarallax  that  should  be  adopted,  but  there  seems  to  be  a  general  converging  of  opinion 
toward  8".S0  as  being,  if  not  the  exact  parallax,  at  least  as  near  it  as  we  are  able  at 
present  to  come.  This  new  determination  of  the  solar  parallax  renders  necessary  a  cor- 
responding change  in  the  planetarj- distances,  etc,  as  the  sun's  distance  is  the  unit  used 
by  astronomers  in  making  all  celestial  measurements.  In  this  chapter,  the  author  has 
followed  the  data  given  by  Prof.  Young  in  his  work  upon  the  Sun,  as  being  the  most 
recent  and  authoritative  view.    (See  p.  280.) 


THE   SUN.  37 

As  we  attempt  to  locate  the  heavenly  bodies  in 
space,  we  are  startled  by  the  enormous  figures  em- 
ployed. The  first  number,  93,000,000  miles,  is  far 
beyond  our  grasp.  Let  us,  however,  try  to  compre- 
hend it.  *  If  there  were  air  to  convey  a  sound  from 
the  sun  to  the  earth,  and  a  noise  could  be  made  loud 
enough  to  pass  that  distance,  it  would  require  over 
fourteen  years  for  it  to  come  to  us.  Supi)Ose  a  raif- 
road  could  be  built  to  the  sun.  An  express-train, 
traveling  day  and  night,  at  the  rate  of  thirty  miles 
an  hour,  would  require  353  years  to  reach  its  destina- 
tion. Ten  generations  would  be  born  and  would  die ; 
the  young  men  would  become  gray -haired,  and  their 
great-grandchildren  would  forget  the  story  of  the 
beginning  of  that  wonderful  journey,  and  would 
read  it  in  history,  as  we  now  read  of  Queen  Elizabeth 
or  of  Shakspere ;  the  eleventh  generation*  would 
see  the  solar  station  at  the  end  of  the  route.  Yet 
this  enormous  distance  of  93,000,000  miles  is  used  as 
the  unit  for  expressing  celestial  distances, — as  the 
foot-rule  for  measuring  space ;  and  astronomers 
speak  of  so  many  times  the  sun's  distance  as  we 
speak  of  so  many  feet  or  inches. 

The  Light  of  the  Sun  is  equal  to  5,563  wax-candles 
held  at  a  distance  of  one  foot  from  the  eye.  It 
would  require  600,000  full-moons  to  produce  a  day  as 
brilliant  as  one  of  cloudless  sunshine,  f 

*  If  a  babe  were  born  with  an  arm  long  enougli  to  reach  the  sun,  and  should  touch 
that  fiery  globe,  the  infant  would  grow  to  manhood  and  to  old  age  and  finally  die,  before 
the  sensation  could  traverse  the  nerve  to  his  brain,  and  he  feel  the  burn. 

t  According  to  Langley,  the  sun  is  blue,  and  to  the  inhabitants  of  other  worlds  may 
shine  as  a  bluer  star  than  Vega.  Tlie  light  from  dill'erent  parts  of  the  solar  disk,  how- 
ever, varies  in  color  :  while  that  from  the  center  has  a  decidedly-blue  tint,  that  from  the 
gd^e  is  of  a  chocolate  hue.    This  difference  is  probably  owing  to  the  fact  that  the  latter 


204827 


38  THE  SOLAR  SYSTEM. 

The  Heat  of  the  Sun. — The  amount  of  heat  we 
receive  annually  is  sufficient  to  melt  a  layer  of  ice 
110  feet  thick,  extending  over  the  whole  earth.*  Yet 
the  sunbeam  is  only  ^o o/o oo  P^r^  ^s  intense  as  it  is  at 
the  surface  of  the  sun.  Moreover,  the  heat  and  light 
stream  off  into  space  equally  in  every  direction.  Of 
this  vast  flood,  only  one  twenty-three-hundred- 
millionth  part  reaches  the  earth. 

If  the  heat  of  the  sun  were  produced  by  the  burn- 
ing of  coal,  it  would  require  a  layer  sixteen  feet  in 
thickness,  extending  over  its  whole  surface,  to  feed 
the  flame  a  single  hour.  Were  the  sun  a  solid  body 
of  coal,  it  would  burn  up  at  this  rate  in  forty-six 
centuries.  Sir  John  Herschel  says  that  if  a  solid 
cylinder  of  ice  45  miles  in  diameter  and  200,000  miles 
long  were  plunged,  end  first,  into  the  sun,  it  would 
melt  in  a  second  of  time. 

Apparent  Size. — The  sun  appears  to  be  a  little  over 
half  a  degree  in  diameter,  so  that  337  solar  disks, 
laid  side  by  side,  would  make  a  half-circle  of  the 
celestial  sphere.  It  seems  a  trifle  larger  to  us  in 
winter  than  in  summer,  as  we  are  3,000,000  miles 
nearer  it.  If  we  represent  the  luminous  surface  of 
the  sun  when  at  its  average  (mean)  distance  by  1,000, 
the  same  surface  will  be  represented  when  in  aphe- 
lion (July)  by  967,  and  when  in  perihelion  (January) 
by  1,034. 


passes  through  a  greater  thickness  of  the  solar  atmosphere,  while  our  o\vn  atmosphere 
floes  its  part  in  strangling  the  blue  rays  of  the  sunlight,  the  red  rays  filtering  through 
with  little  loss. 

*  Recent  experiments  by  Langley  seem  to  increase  this  estimate  to  that  of  a  sheet 
of  jce  180  feet  thick  (Popular  Science  Monthly,  Sept.,  1885). 


THE    SUN. 


39 


Dimensions.— Its  diameter  \^  about  866,000  miles.* 
Let  us  try  to  understand  this  amount  by  comparison. 

A  mountain  upon  the  surface  of  the  sun,  to  bear 
the  same  proportion  to  the  globe  itself  as  the  loftiest 
peak  of  the  Himalayas  does  to  the  earth,  would  need 
to  be  about  600  miles  high. 

Again  :  Suppose  the  sun  were  hollow,  and  the 
earth,  as  in  the  cut  (Fig.  8),  placed  at  the  center,  not 


Fig.  8. 


only  would  there  be  room  for  the  moon  to  revolve  in 
its  regular  orbit  within  the  shell,  but  that  would 


*  Pythagoras,  whose  theory  of  the  universe  was  in  so  many  respects  very  like  the 
one  we  receive,  believed  the  sun  to  be  44,000  miles  from  the  earth,  and  seventy-five 
miles  in  diameter. 


40  THE  SOLAR  SYSTEM. 

stretch  off  in  every  direction  nearly  200,000  miles 
beyond. 

Its  volume  is  1,300,000  times  that  of  the  earth — 
i.  e.,  it  would  take  1,300,000  earths  to  make  a  globe 
the  size  of  the  sun.  Its  mass  is  750  times  that  of 
all  the  planets  and  moons  in  the  solar  system,  and 
330,000  times  that  of  the  earth.  Its  weight  may  be 
expressed  in  tons,  thus  : 

1,910,278,070,000,000,000,000,000,000.* 

The  Density  of  the  sun  is  only  about  one-fourth 
that  of  the  earth,  or  1.41  that  of  water,  so  that  the 
weight  of  a  body  transferred  from  the  earth  to  the 
sun  would  not  be  increased  in  proportion  to  the  com- 
parative size  of  the  two.  On  account  also  of  the  vast 
size  of  the  sun,  its  surface  is  so  far  from  its  center 
that  the  attraction  is  largely  diminished,  since  that 
decreases,  we  remember,  as  the  square  of  the  dis- 
tance. However,  a  man  weighing  at  the  earth's 
equator  150  lbs.,  at  the  sun's  equator  would  weigh 
about  two  tons, — a  force  of  attraction  that  would  in- 
stantly crush  him.  At  the  earth's  equator,  a  stone 
falls  16  feet  the  first  second  ;  at  the  sun's  equator,  it 
would  fall  144  feet,  f 

Telescopic  Appearance  of  the  Sun :  Sun  Spots. — 
We  may  sometimes  examine  the  sun  at  early  morn- 
ing or  late  in  the  afternoon  with  the  naked  eye,  and 

*  This  number  is  meaningless  to  our  imagination,  but  yet  it  represents  a  force  of 
attraction  that  holds  our  own  earth  and  all  the  planets  steadily  in  their  places;  while  it 
fills  the  mind  with  an  indescribable  awe  as  we  think  of  that  Being  who  "made  the  sun, 
and  holds  it  in  the  very  palm  of  His  hand." 

t  A  singular  consequence  of  this  has  been  suggested.  "A  cannon-ball  could  be 
thrown  only  a  short  distance,  since  it  would  pass  through  a  path  of  great  curvature,  9n^ 
lyould  fall  to  the  sun  within  a  few  yards  of  the  gun." 


THE  StfN. 


41 


at  midday  by  using  a  smoked  glass.  The  disk  will 
appear  distinct  and  circular,  and  with  no  spot  to  dim 
its  brightness.     If  we  use  a  telescope  of  moderate 


Fig  0 


The  Swn  seen  through  a  Telescope. 


power,  taking  the  precai^tion  to  shield  the  eye  with 
a  colored  eye-piece,  we  shall  find  the  surface  of  the 
sun  sprinkled  with  irregular  spots  (Fig.  9).* 

•  Tlie  natural  purity  of  the  sun  seems  to  have  been  formerly  an  article  of  faith  among 
astronomers,  and  therefore  on  no  account  to  be  called  in  question.  Seheiner,  it  is  said, 
having  reported  to  his  superior  that  he  had  seen  spots  on  the  sun's  face,  was  abruptly 
dismissed  with  these  remarks  :  "  I  have  read  ^Vristotle's  WTitings  from  end  to  end  many 
times,  and  I  assure  you  I  do  not  find  anything  in  them  similar  to  that  which  you  men- 
tion.  Go,  my  son,  tianquillize  yourself ;  be  assured  tliat  what  you  take  for  spots  are  the 
faults  of  your  glasses  or  your  own  eyes." 


i2  THE  SOLAR  SYSTEM. 

Discovery  of  the  Solar  Spots.— The  solar  spots 
seem  to  have  been  noticed  as  early  as  807  a.d.,  al- 
though the  telescope  was  not  invented  until  1610, 
and  Galileo  is  considered  to  have  discovered  them  in 
the  following  year.  * 

Number  and  Location. — Sometimes,  but  rarely, 
the  sun's  disk  is  clear.  During  a  period  of  ten  years, 
observations  were  made  on  1982  days,  on  372  of 
which  there  were  no  spots  seen.  As  many  as  two 
hundred  spots  have  been  noticed  at  one  time.  They 
are  mostly  found  in  two  belts,  one  on  each  side  of 
the  equator,  within  not  less  than  10"  nor  more  than 
SO''  of  latitude.  They  seem  to  herd  together, — the 
length  of  the  straggling  group  being  generally  par- 
allel to  the  equator. 

Size  of  the  Spots. — It  is  not  uncommon  to  find  a 
spot  with  a  surface  larger  than  that  of  the  earth. 
Schroter  measured  one  more  than  29,000  miles  in 
diameter.  Sir  J.  W.  Herschel  calculated  that  one 
which  he  saw  was  50,000  miles  in  diameter.  In 
1843,  one  was  seen  which  was  75,000  miles  across, 
and  was  visible  to  the  naked  eye  for  an  entire  week,  f 
On  the  day  of  the  eclipse  in  1858,  a  spot  over  108,000 
miles  broad  was  distinctly  seen,  and  attracted  gen- 
eral attention  in  this  country.  In  1839,  Captain  Davis 
saw  one  which  he  computed  was  180,000  miles  long, 
and  had  an  area  of  twenty-four  billion  square  miles. 

If  these  are  deep  openings  in  the  luminous  atmos- 

*  We  read  in  the  l(^-book  of  tlie  good  ship  Richard  of  Arundell,  on  a  voyage,  in 

1590,  to  the  coast  of  Guinea,  that  "  on  the  7,  at  the  going  downe  of  the  snnne,  we  saw  a 

^reat  black  spot  in  the  sunne  ;  and  the  8  day,  both  at  rising  and  setting,  we  saw  the  like, 

-which  si>ot  to  me  seeming  was  about  the  bignesse  of  a  shilling,  being  in  5  degrees  of 

ititude,  and  still  there  came  a  great  billow  out  of  the  souther  board." 

t  1''  en  the  sun  s  surface  =  430.3  miles.    This  spot  was  2'47"  acrosa  {Scktoabt}. 


THE  SUN. 


43 


Fig.  10. 


phere  of  the  sun,  what  an  abyss  must  that  be  at  "  the 
bottom  of  which  our  earth  could  lie  like  a  boulder  in 
the  crater  of  a  volcano  ! " 

Spots  Consist  of  Distinct  Paets. — From  the  ac- 
companying repre- 
sentation, it  will  be 
seen  that  the  spots 
generally  consist 
of  one  or  more  dark 
portions  called  the 
umbra,  and  around 
that  a  grayish  por- 
tion styled  the  pe- 
numbra {pene,  al- 
most, and  umbray 
black).  Sometimes, 
however,  umbrae 
appear  without  a 
penumbra,andvice 
versa.  The  umbra  itself  has  generally  a  dense  black 
center,  called  the  nucleus.  Besides  this,  the  umbra 
is  sometimes  divided  by  luminous  bridges. 

Spots  are  in  Motion. — The  spots  change  from  day 
to  day  ;  but  all  have  a  common  movement.  About 
fourteen  days  are  required  for  a  spot  to  pass  across 
the  disk  of  the  sun  from  the  eastern  side,  or  limb,  to 
the  western  ;  in  fourteen  days,  it  reappears,  changed 
in  form  perhaps,  but  generally  recognizable. 

Spots  apparently  Change  their  Speed  and  Form 

AS  THEY   PASS  ACROSS  THE   DiSK. — A  Spot   is    SCCU    OU 

the  eastern  limb  ;  day  by  day  it  progresses,  with  a 
gradually-increasing  rapidity,  until   it  reaches  the 


&un-Spots. 


44  THE  SOLAR  SYSTEM. 

center ;  it  then  slowly  loses  its  rapidity,  and  finally 
disappears  on  the  western  limb.  The  diagram  illus- 
trates the  apparent  change  which  takes  place  in  the 
form.  Suppose  at  first  the  spot  is  of  an  oval  shape  ; 
as  it  approaches  the  center  it  apparently  widens  and 
becomes  circular.  Having  passed  that  point,  it  be- 
comes more  and  more  oval  until  it  disappears. 

Fig.  11. 


Change  in  Spots  as  they  Cross  the  Disk. 

This  change  in  the  Spots  proves  the  Sun's  Rota- 
tion ON  ITS  Axis. — These  changes  can  be  accounted 
for  only  on  the  supposition  that  the  sun  rotates  on 
its  axis  :  indeed,  they  are  the  precise  effects  which 
the  laws  of  perspective  demand  in  that  case.  About 
twenty-seven  days  elapse  from  the  appearance  of  a 
spot  on  the  eastern  limb  before  it  is  seen  a  second 
time.  During  this  period  the  earth  has  gone  forward 
in  its  orbit,  so  that  the  location  of  the  observer  is 
changed ;  allowing  for  this,  the  sun's  time  of  rotation 
at  the  equator  is  about  twenty-five  days  (25  d.,  8  h., 
10  m.  :  Langier), 


THE  StTN. 


45 


Curiously  enough,  the  equatorial  regions  move 
more  rapidly,  and  complete  a  rotation  in  less  time, 
than  the  rest  of  the  siin.  While  a  spot  near  the 
equator  performs  a  rotation  in  twenty-seven  days, 
one  situated  half- 
way to  either  pole, 
requires  nearly 
twenty -eight  days. 

Synodic  and  side- 
real REVOLUTIONS 
OF  THE    SPOTS. — We 

can  easily  under- 
stand why  we  make 
an  allowance  for 
the  motion  of  the 
earth  in  its  orbit. 
Suppose  a  solar  spot 
at  a,  on  a  line  pass- 
ing from  the  center 
of  the  earth  to  the 
center  of  the  sun. 
For  the  spot  to  pass 
around  the  sun  and 
come  into  that  same 


Synodic  and  Sidereal  Revolutions. 


position  again,  requires  about  twenty-seven  days. 
But,  during  this  time,  the  earth  has  passed  on  from 
T  to  T'.  The  spot  has  not  only  traveled  around  to 
a  again,  but  also  beyond  that  to  a',  or  the  distance 
from  a  to  a'  more  than  an  entire  revolution.  To  do 
this,  requires  about  two  days.  A  revolution  from  a 
around  to  a'  is  called  a  synodic,  and  one  from  a 
around  to  a  again  is  called  a  sidereal,  revolution. 


46  the  solar  system. 

Spots  do  not  always  move  in  straight  lines. — 
Sometimes  their  path  curves  toward  the  north,  and 

Fig.  IS. 


March.  June.  September. 

sometimes  toward  the  south,  as  in  the  figure.  This 
can  be  explained  only  on  the  supposition  that  the 
sun's  axis  is  inclined  to  the  ecliptic  (7""  15'). 

Spots  have  a  motion  of  their  own. — Besides  the 
motion  already  named  as  assigned  to  the  sun's  rota- 
tion, nearly  every  spot  seems  to  have  an  individual 
motion.  Some  spots  circle  about  in  small  elliptical 
paths,  often  quite  regularly  for  weeks  and  even 
months.  Immense  cyclones  occasionally  pass  over 
the  surface  with  fearful  rapidity,  producing  rotation 
and  sudden  changes  in  the  spots.  At  other  times, 
however,  the  spots  seem  "to  set  sail  and  move 
across  the  disk  of  the  sun  like  gondolas  over  a  silver 
sea." 

Spots  change  their  real  form. — Spots  break  out 
and  then  disappear  under  the  eye  of  the  astronomer. 
WoUaston  saw  one  that  seemed  to  be  shattered  like 
a  fragment  of  ice  when  it  is  thrown  on  a  frozen 
surface,  breaking  into  pieces,  and  sliding  off  in 
every  direction.  Sometimes  one  divides  itself  into 
several  nuclei,  while  again  several  nuclei  combine 


THE  SUN. 


47 


into  a  single  nucleus.  Occasionally  a  spot  will  re- 
main for  six  or  eight  rotations,  while  often  it  will 
last  scarcely  half  an  hour.     Sir  W.  Herschel  relates 


Fig.  lU. 


Solar  Cyclone,  May  5th,  1857.     (Secchi.) 

that,  when  examining  a  spot  through  his  telescope, 
he  turned  away  for  a  moment,  and  on  looking  back 
it  was  gone. 

Appearance  of  the  spots  is  periodical.* — It  is 
a  remarkable  fact  that  the  number  of  spots  increases 
and  diminishes  through  a  regular  interval  of  about 
11.11  years.  These  periodic  variations  are  closely 
connected  with  similar  variations  in  the  aurora 
and  magnetic  earth-currents  which  interfere  with 
the  telegraph. 

Are  the  spots  influenced  by  the  planets  ? — 


*  The  regular  increase  and  diminution  in  the  number  of  the  spots  was  discovered 
by  Schwabe  of  Prussia,  who  watched  the  sun  so  carefully  that  it  is  said  "for  thirty  years 
the  sun  never  appeared  above  the  horizon  without  being  confronted  by  his  imperturbable 
telescope." 


48  The  solar  system. 

Many  astronomers  of  high  standing  believe  that  the 
solar  spots  are  especially  sensitive  to  the  approach 
of  Mercury  and  Venus,  on  account  of  their  nearness, 
and  of  Jupiter,  because  of  its  size  ;  that  the  area  of 
the  spots  exposed  to  view  from  the  earth  is  uniformly 
greatest  when  any  two  of  the  larger  planets  come 
into  line  with  the  sun ;  and  that  when  both  Venus 
and  Jupiter  are  on  the  side  of  the  sun  opposite  to  us, 
the  spots  are  much  larger  than  when  Venus  alone  is 
in  that  position.  Most  authorities,  however,  doubt 
the  accuracy  of  these  observations,  and  deny  this 
planetary  influence  altogether. 

Spots  do  not  influence  fruitfulness  of  the 
SEASON. — Herschel  first  advanced  the  idea  that  years 
of  abundant  spots  would  be  years  also  of  plentiful 
harvest.  This  is  not  now  generally  received.  What 
two  years  could  be  more  dissimilar  than  1859  and 
1860  ?  Both  abounded  in  solar  spots,  yet,  in  Europe, 
one  was  a  fruitful  year  and  the  other  one  of  almost 
famine.  Whether  the  spots  influence  the  weather  is 
still  a  mooted  question. 

Spots  are  cooler  than  the  surrounding  sur- 
face.— It  seems  that  the  breaking  out  of  a  spot  sen- 
sibly diminishes  the  temperature  of  that  portion  of 
the  sun's  disk.  The  faculae,  on  the  other  hand,  do 
not  increase  the  temperature  {Secchi). 

Spots  are  depressions.  —  Careful  observations 
show  that,  in  general,  the  "  floor,"  so  to  speak,  of  the 
umbra  is  sunk  from  two  to  six  thousand  miles  below 
the  level  of  the  luminous  surface  {Young). 

Comparative  brightness  of  spots  and  sun. — 
If    we    represent    the  ordinary    brightness    of    the 


49 


Photograxihic  View  of  Spots  and  Faculte. 


sun  by  1,000,  then  that  of  the  penumbra  would 
be  about  800,  and  that  of  the  umbra,  540  (Lang- 
ley).  There  may  be 
much  light  and  heat 
radiated  by^a  spot, 
which  seems  black  as 
compared  with  the  sun  ; 
for  we  remember  that 
even  a  calcium  light, 
held  between  our  eyes 
and  the  sun,  appears 
as  a  black  spot  on  the 
disk  of  that  luminary. 

Appearance  of  the 
sun's  surface, — Even  a 
telescope  of  moderate 
power  will  show  the 
?5urface  of  the  sun  to  have  a  peculiar  mottled  appear' 
3 


Fig.  16. 

'-"'"' '^^"" 

■S:-tS3^::rj 

I 

^ 

^ 

1 

'^^ 

g 
P 

' 

''k 

FaculcB. 


50 


THE  SOLAR  SYSTEM. 


ance  not  unlike  that  of  an  orange  skin.  But,  under 
favorable  circumstances  and  with  a  telescope  of  high 
power,  the  solar  disk  is  found  to  be  covered  with 
small,  intensely  bright  bodies  irregularly  distributed. 


Fig.  17. 


WiOov-Leaf. 

These  are  now  known  as  rice-grains.'*'  They  are 
often  apparently  crowded  together  in  luminous 
ridges,  or  streaks,  termed  faculce  {facula,  a  torch)  ; 
while  the  rice-grains  themselves,  according  to  Prof. 
Langley,  are  composed  of  granules.     Minute  as  a 

*  Various  obsen-ers  describe  the  solar  surface  differently.  A  peculiar,  elongated, 
leaf-shaped  appearance  of  the  rice-grains,  called  the  willow-leaf  structure,  is  shown  in 
Fig.  17,  as  seen  by  Nasmyth.  Newcomb  compares  the  sun's  appearance  to  that  of  a 
plate  of  rice-soup.  Young  says  it  frequently  resembles  bits  of  straw  lying  parallel  to 
one  another— the  '•  thatched-straw  formation," 


Typical  Sun-spot,  uj  Dec.  1S73,  showing  the  filaments  pointing  to  the  center. 


THE  SUN.  61 

granule  seems,  probably  the  smallest  has  a  diameter 
of,  at  least,  100  miles. 

Physical  Constitution  of  the  Sun.* — Of  the  consti- 
tution of  the  sun,  and  the  cause  of  the  solar  spots, 
very  little  is  definitely  known. 

Wilson's  Theory  supposed  that  the  sun  is  com- 
posed of  a  solid,  dark  globe,  surrounded  by  three 
atmospheres.  The  first,  nearest  the  black  body  of 
the  sun,  is  a  dense,  cloudy  covering,  possessing  high 
reflecting  power.  The  second  is  called  the  photo- 
sphere. It  consists  of  an  incandescent  gas,  and  is 
the  seat  of  the  light  and  heat  of  the  sun,  being  the 
sun  that  we  see.  The  third,  or  *  outer  one,  is  trans- 
parent— very  like  our  atmosphere. 

According  to  this  theory,  the  spots  are  to  be  ex- 
plained in  the  following  manner.  They  are  simply 
openings  in  these  atmospheres  made  by  powerful 
upward  currents.  At  the  bottom  of  these  chasms, 
we  see  the  dark  sun  as  a  nucleus  at  the  center,  and 
around  this  the  cloudy  atmosphere — the  penumhra. 
This  explains  a  black  spot  with  its  penumbra.  Some- 
times the  opening  in  the  photosphere  may  be  smaller 
than  that  in  the  inner  or  cloudy  atmosphere  ;  in  that 
case  there  will  be  a  black  spot  without  a  penumbra. 

It  will  be  natural  to  suppose  that  when  the  heated 
gas  of  the  photosphere,  or  second  atmosphere,  is 
violently  rent  asunder  by  an  eruption  or  current 
from  below,  luminous  ridges  will  be  formed  by  the 
heaped-up  gas  on  every  side  of  the  opening.  This 
would  account  for  the  faculce  surrounding  the  sun- 

*  For  the  views  of  various  authorities  on  the  constitution  of  the  sun,  solar  spots,  etc., 
see  Newcorab's  Astronomy,  third  edition,  p.  271, 


52 


THE  SOLAR  SYSTEM. 


spots.  It  will  be  natural,  also,  to  suppose  that  some- 
times the  cloudy  atmosphere  below  will  close  up  first 
over  the  dark  surface  of  the  sun,  leaving  only  an 
opening  through  the  photosphere,  disclosing  at  the 
bottom  a  grayish  surface  of  penumbra.    We  can 


Fig.  19. 


WiUon's  Theory. 

readily  see,  also,  how,  as  the  sun  revolving  on  its 
axis  brings  a  spot  nearer  and  nearer  to  the  center, 
thus  giving  us  a  more  direct  view  of  the  opening,  we 
can  see  more  and  more  of  the  dark  body.  Then  as 
it  passes  by  the  center  the  nucleus  will  disappear, 


THE  SUN.  53 

until  finally  we  can  see  only  the  side  of  the  fissure, 
the  penumbra,  which,  in  its  turn,  will  vanish. 

The  Present  Theory*  is  deduced  from  the  re- 
sults of  Spectrum  Analysis,  of  which  we  shall  here- 
after speak.  It  is  constantly  being  modified  by  new 
discoveries.  But  we  may,  in  general,  believe  the 
sun  to  be  a  vast,  fiery  body,  surrounded  by  an 
atmosphere  of  substances  volatilized  by  the  intense 
heat.  Among  these,  we  recognize  familiar  elements, 
as  iron,  copper,  &c. 

The  different  portions  of  the  sun  are  thought  to  be 
arranged  thus  :  (1).  The  nucleus,  probably  gaseous  ;  \ 
(2).  The  photosphere,  an  envelope  several  thousand 
miles  thick,  which  constitutes  the  visible  part  of  the 
sun  ;  (3).  The  chromosphere,  composed  of  luminous 
gas,  mostly  hydrogen,  and  the  seat  of  enormous  pro- 
tuberances, tongues  of  fire,  which  dart  forth,  some- 
times at  the  rate  of  150  miles  per  second,  and  to  a 
distance  of  over  100,000  miles  ;  (^).  The  corona,X  an 
outer  appendage  of  faint,  pearly  light,  consisting  of 
streamers  reaching  out  often  several  hundred  thou- 
sand miles.  Of  these  solar  constituents,  the  eye  and 
the  telescope  ordinarily  reveal  only  the  photosphere  ; 
the  rest  are  seen  during  a  total  eclipse  or  by  means 
of  the  spectroscope. 

The  outer  portion  of  the  sun  radiates  its  heat  and 

*  As  Kirchhoff,  by  his  discoveries  in  Spectrum  Analysis,  laid  the  foundation  of  this 
theory,  it  is  often  called  after  him. 

t  The  interior  of  the  sun,  if  gaseous,  must  be  powerfully  condensed,  because  of  the 
tremendous  pressure  of  the  atmosphere.  The  high  temperature,  however,  prevents  the 
gas  from  liquefying.  Tlie  rain-.storms  on  the  sun,  if  such  ever  occur,  consist  of  drops 
of  molten  iron,  copper,  zinc,  &c.,  vaporized  by  the  enormous  heat ;  and  often  a  tempest 
would  drive  before  it  this  white-hot,  metallic  blast,  with  a  speed  of  100  miles  per  second. 

t  This  is  so  called  because,  during  a  total  eclipse,  it  forms  around  the  moon  a  corona, 
or  glory,  that  is  the  most  wonderful  feature  of  this  rare  event.    (See  p.  141.) 


54  THE  SOLAR  SYSTEM. 

light,  and,  becoming  cooler,  sinks  ;  the  hotter  matter 
in  the  interior  then  risQS  to  take  its  place,  and  thus 
convection  currents  are  established  (Physics,  p.  193). 
The  cooler,  descending  currents  are  darker,  and  the 
hotter,  ascending  ones  are  lighter  ;  this  gives  rise  to 
the  mottled  look  of  the  sun.  At  times,  this  occurs  on 
a  grand  scale,  and  the  heated,  up-rushing  masses 
form  the  faculse,  and  the  cooler,  down-rushing  ones 
produce  the  solar  spots. 

The  Heat  of  the  Sun  is  generally  considered  to  be 
produced  by  condensation,  whereby  the  size  of  the 
sun  is  constantly  decreasing,  and  its  potential  energy 
thus  converted  into  kinetic.  The  dynamic  theory 
accounts  for  the  heat  and  the  solar  spots  by  assum- 
ing that  there  are  vast  numbers  of  meteors  revolving 
around  the  sun,  and  that  these  constantly  rain  down 
upon  the  surface  of  that  luminary.  *  Their  motion, 
thus  stopped,  is  changed  to  heat,  and  feeds  this  great 
central  fire.  Were  Mercury  to  strike  the  sun  in  this 
way,  it  would  generate  sufficient  heat  to  compensate 
the  loss  by  radiation  for  seven  years. 

Doubtless,  the  solar  heat  is  gradually  diminishing, 
and  will  ultimately  be  exhausted.  In  time,  the  sun 
will  cease  to  shine,  as  the  earth  did  long  since.  New- 
comb  says  that  in  5,000,000  years,  at  the  present 
rate,  the  sun  will  have  shrunk  to  half  its  present 
size,  and  that  it  cannot  sustain  life  on  the  earth 
more  than  10,000,000  years  longer.  Of  this  we  may 
be  assured,  there  is  enough  to  support  life  on  our 
globe  for  millions  of  years  yet  to  come. 

*  Tlic  heat  of  the  sun  could  be  maintained  by  an  annual  contraction  of  220  feet  in 
its  diameter,  a  decrease  so  insignificant  as  to  be  imperceptible  with  the  best  instru- 
ments ;  or  by  the  annual  impact  of  meteors  equal  in  amount  to  y  the  mass  of  Mercury. 


THE  PLANETS.  55 


II.— THE     PLANETS. 

INTRODUCTION. 

The  Planets  will  be  described  in  regular  order, 
passing  outward  from  the  sun.  In  this  journey,  we 
shall  examine  each  planet  in  turn,  noticing  its  dis- 
tance, size,  length  of  year,  duration  of  day  and 
night,  temperature,  climate,  number  of  moons,  and 
other  interesting  facts,  showing  how  much  we  can 
know  of  its  world-life  in  spite  of  its  wonderful  dis- 
tance. We  shall  encounter  the  earth  in  our  imag- 
inary wanderings  through  space,  and  shall  explain 
many  celestial  phenomena  already  partially  familiar 
to  us. 

In  all  these  worlds,  we  shall  find  traces  of  the 
same  Divine  hand,  molding  and  directing  in  con- 
formity to  one  universal  plan.  We  shall  discover 
that  the  laws  of  light  and  heat  are  invariable,  and 
that  the  force  of  gravity,  which  causes  a  stone  to  fall 
to  the  ground,  acts  similarly  upon  the  most  distant 
planet.  Even  the  elements  of  which  the  planets  are 
composed  will  be  familiar  to  us,  so  that  a  book  of 
natural  science  published  here  might,  in  its  general 
features,  answer  for  use  in  a  school  on  Mars  or 
Jupiter. 

Common  Characteristics  (Hind). — 1.  The  planets 
move  in  the  same  direction  around  the  sun ;  their 


56  THE  SOLAR  SYSTEM. 

course,  as  viewed  from  the  north  side  of  the  ecliptic, 
being  contrary  to  the  motion  of  the  hands  of  a 
watch. 

2.  The}^  describe  elliptical  paths  around  the  sun, — 
not  differing  much  from  circles. 

3.  Their  orbits  are  more  or  less  inclined  to  the 
ecliptic,  and  intersect  it  in  two  points — the  nodes, — 
one-half  of  the  orbit  lying  north,  and  the  other  south 
of  the  earth's  path. 

■i.  They  are  opaque  bodies,  and  shine  by  reflecting 
the  light  they  receive  from  the  sun. 

5.  They  rotate  upon  their  axes  in  the  same  way 
as  the  earth.  Tliis  we  know  by  telescopic  observa- 
tion to  be  the  case  with  many  planets,  and  by  anal- 
ogy the  rule  may  be  extended  to  all.  Hence,  they 
have  the  alternation  of  day  and  night. 

6.  Agreeably  to  the  principles  of  gravitation,  their 
velocity  is  greatest  at  that  part  of  their  orbit  nearest 
the  sun,  and  least  at  that  part  most  distant  from  it ; 
in  other  words,  they  move  quickest  in  perihelion,  and 
slowest  in  aphelion. 

Comparison  cf  the  two  G-roups  of  the  Major 
Planets.  [Chambers.) — Separating  the  major  planets 
into  two  groups,  if  we  take  Mercury,  Venus,  the 
Earth,  and  Mars  as  belonging  to  the  interior,  and 
Jupiter,  Saturn,  Uranus,  and  Neptune  to  the  exterior 
group,  we  shall  find  that  they  differ  in  the  following 
respects : 

1.  The  interior  planets,  with  the  exception  of  the 
Earth  and  Mars,  are  not  attended  by  any  satellite, 
while  all  the  exterior  planets  have  satellites. 

2.  The  average  density  of  the  first  group  consider- 


i'HE  PLANETS. 


57 


ably  exceeds  that  of  the  second,  the  approximate 
ratio  being  5:1. 

3.  The  mean  duration  of  the  axial  rotations,  or  the 
mean  length  of  the  day  of  the  interior  planets,  is 
much  longer  than  that  of  the  exterior  ;  the  average 
in  the  former  case  being  about  twenty-four  hours, 
but  in  the  latter  only  about  ten  hours. 

Properties  of  the  Ellipse. — In  Fig.  20,  S  and  S'  are 
the  foci  of  the  ellipse ;  A  C  is  the  major  axis  ;  B  D, 
the  minor  or  conjugate  axis;  O,  the  center:  or, 
astronomically,  O  A  is  the  semi-axis-major  or  mean 


An  Ellipse. 


distance,  O  B  the  semi-axis-minor :  the  ratio  of  O  S 
to  O  A  is  the  eccentricity  ;  the  least  distance,  S  A,  is 
the  perilielion  distance  ;  the  greatest  distance,  S  C, 
the  aphelion  distance. 

Characteristics  of  a  Planetary  Orbit. — It  will  not 
be  difficult  to  follow  in  the  mind  the  additional 
characteristics  of  a  planet's  orbit.  Take  two  hoops, 
and  bind  them  into  an   oval  shape.      Incline  one 


58  THE  SOLAR  SYSTEM. 

slightly  to  the  other,  as  shown  in  Fig.  21.  Let  the 
horizontal  hoop  represent  the  ecliptic.  Imagine  a 
planet  following  the  inclined  hoop,  or  ellipse ;  at  a 
certain  point  it  rises  above  the  level  of  the  ecliptic  :  * 
this  point  is  called  the  ascending  node,  and  the  op- 
Fig.  SI. 


Planetary  Orbits. 

posite  point  of  intersection  is  termed  the  descending 
node.  A  line  connecting  the  two  nodes  is  the  line  of 
the  nodes.  The  longitude  of  the  node  is  its  distance 
from  the  first  point  of  Aries,  measured  on  the  eclip- 
tic, eastward. 

Comparative  Size  of  Planets  (Chambers). — The  following  scheme 
will  assist  in  obtaining  some  notion  of  the  magnitude  of  the  planetary 
system.  Choose  a  level  field  or  common  ;  on  it  place  a  globe  two  feet 
in  diameter  for  the  Snn  :  Vulcan  will  then  be  represented  by  a  small 
pin's  head,  at  a  distance  of  about  twenty-seven  feet  from  the  center 
of  the  ideal  sun  ;  Mercury  by  a  mustard-seed,  at  a  distance  of  eighty-two 
feet ;  Venus  by  a  pea,  at  a  distance  of  142  feet ;  the  Earth,  also,  by  a  pea, 
at  a  distance  of  215  feet ;  Mars  by  a  small  pepper-corn,  at  a  distance  of 
327  feet ;  the  minor  planets  by  grains  of  sand,  at  distances  varying  from 
500  to  600  feet.     If  space  wiU  permit,  we  may  place  a  moderate-sized 


*  Lockyer  beautifully  says :  "  We  may  imagine  the  earth  floating  aroiind  the  sun  on 
a  boundless  ocean,  both  sun  and  earth  being  half  immersed  in  it.  This  level,  this  plane, 
the  plane  of  the  ecliptic  (because  all  eclipses  occur  in  it),  is  used  by  astronomers  as  we 
use  the  sea-level.  We  say  a  mountain  is  so  far  above  the  level  of  the  sea.  The  astrono- 
mer says  the  star  is  so  high  above  the  level  of  the  ecliptic 


THE  PLANETS. 


59 


orange  nearly  one-quarter  of  a  mile  distant  from  the  starting  point  to  rep- 
resent Jupiter ;  a  small  orange  two-fifths  of  a  mUe  for  Saturn  ;  a  full-sized 
cherry  three-quarters  of  a  mile  distant  for  Uranus ;  and  lastly,  a  plum 
1|  miles  off  for  Neptune,  the  most  distant  planet  yet  known.     Extending 


Fig,  ss. 


Comparative  Size  of  the  rUinets. 


this  scheme,  we  should  find  that  the  aphelion  distance  of  Encke's  comet 
would  be  at  880  feet ;  the  ajjhelion  distance  of  Donati's  comet  of  1858  at 
six  miles  ;  and  the  nearest  fixed  star  at  7,500  miles. 


60 


THE  SOLAR  SYSTEM. 


According  to  this  scale,  the  daily  motion  of  Vulcan  in  its  orbit  would  be 
4|  feet;  of  Mercury,  3  feet  ;  of  Venus,  2  feet ;  of  the  Earth,  1|  feet ;  of 
Mars,  1^  feet ;  of  Jupiter,  10|  inches  ;  of  Saturn,  7^  inches  ;  of  Uranus,  5 
inches  ;  and  of  Neptune,  4  inches.  This  illustrates  the  fact  that  the  orbital 
velocity  of  a  planet  decreases  as  its  distance  from  the  sun  increases.  * 

Conjunction  of  Planets. — The  grouping  together 
of  two  or  more  planets  within  a  limited  area  of  the 
heavens  is  a  rare  event.  The  earliest  record  we 
have  is  the  one  of  Chinese  origin  (p.  G),  stating  that 
a  conjunction  of  Mars,  Jupiter,  Saturn,  and  Mercury 

Fig.  23. 


■r  in  Conjunction,  January  SO,  ISOi. 


occurred  in  the  reign  of  the  Emperor  Chuenhio. 
Astronomers  tell  us  that  this  took  place  Feb.  28,  244G 
B.  c,  between  10°  and  18^  of  Pisces.  There  is  a  very 
general  impression,  however,  that  this  conjunction 
was  afterward  calculated  and  chronicled  in  their 
records.      In    1725,   Venus,   Mercury,  Jupiter,  and 

♦  If  we  accept  the  Xelmlar  Hj-pothesis  (p.  255),  we  can  easily  understand  the  reason 
of  this;  the  exterior  planets,  being  made  earlier,  had  the  motion  of  the  nebula  during 
its  earlier  stage.  The  rotation-velocity  of  the  nebula  kept  increasing,  and  so,  of  course, 
each  planet  Dossessed  a  higher  rate  of  orbital  speed  than  the  preceding  one- 


tHE   PLANETS.  61 

Mars  appeared  in  the  same  field  of  the  telescope.  In 
1859,  Venus  and  Jupiter  came  so  near  each  other 
that  they  appeared  to  the  naked  eye  as  one  object. 

Are  the  Planets  Inhabited? — This  question  is  one 
which  very  naturally  arises,  when  we  think  of  the 
planets  as  worlds  in  so  many  respects  similar  to  our 
own.  We  can  give  no  satisfactory  answer.  Many 
think  that  the  only  object  God  can  have  in  making 
a  world  is  to  form  an  abode  for  man.  Our  own  earth 
was  evidently  fitted  up,  although  perhaps  not  cre- 
ated, for  this  express  purpose.  Everywhere  about 
us  we  find  proofs  of  special  forethought  and 
adaptation.  Coal  and  oil  in  the  earth  for  fuel  and 
light,  forests  for  timber,  metals  in  the  mountains 
for  machinery,  rivers  for  navigation,  and  level 
plains  for  corn.  The  human  body,  the  air,  light,  and 
heat  are  all  fitted  to  one  another  with  exquisite 
nicety. 

When  we  turn  to  the  planets,  we  do  not  know  but 
God  has  other  races  of  intelligent  beings  who  inhabit 
them,  or  even  entirely  different  ends  to  attain.  Of 
this,  however,  we  are  assured,  that,  if  inhabited,  the 
conditions  on  which  life  is  supported  vary  much 
from  those  familiar  to  us.  When  we  come  to  speak 
of  the  different  planets,  we  shall  see  (1)  how  they 
differ  in  light  and  heat,  from  seven  times  our  usual 
temperature  to  less  than  -^^^^ ;  (2)  in  the  intensity 
of  the  force  of  gravity,  from  2^  times  that  of 
the  earth  to  less  than  | ;  (3)  in  the  constitution  of 
the  planet  itself,  from  a  density  ^  heavier  than  that 
of  the  earth  to  one  nearly  that  of  cork. 

The    temperature    may    often    sweep    downward 


6^ 


THE  SOLAR  SYSTEM. 


through  a  scale  of  2,000°  in  passing  from  Mercury 
to  Neptune.  No  human  being  could  reside  on  the 
former,  while  we  cannot  conceive  of  any  polar  inhab- 


Fig.  %. 


Size  of  tht'  .S(t/t  ay  seen  frtrni  tke  V  in  nets 


itaat  who  could  endure  the  intense  cold  of  the  latter. 
At  the  sun,  one  of  our  pounds  would  weigh  over  27 
pounds ;  on  our  moon,  the  pound  weight  would  be- 


fHE  PLANETS.  63 

Come  only  about  two  ounces  ;  while  on  Vesta,  one  of 
the  planetoids,  a  man  could  easily  spring  sixty  feet 
in  the  air  and  sustain  no  shock  in  falling.  Yet,  while 
we  speak  of  these  peculiarities,  we  do  not  know 
what  modification  of  the  atmosphere  or  physical 
features  may  exist  on  Mercury  to  temper  the  heat, 
or  on  Neptune  to  reduce  the  cold. 

With  all  these  diversities,  we  must,  however,  admit 
the  power  of  an  all-wise  Creator  to  form  beings 
adapted  to  the  life  and  the  land,  however  different 
from  our  own.  The  Power  that  prepared  a  world 
for  us,  could  as  easily  and  perfectly  prepare  one  for 
other  races.  May  it  not  be  that  the  same  love  of 
diversity,  that  will  not  make  two  leaves  after  the 
same  pattern  nor  two  pebbles  of  the  same  size,  de- 
lights in  worlds  peopled  by  races  as  diverse  ?  * 

While,  then,  we  cannot  affirm  that  the  planets  are 
inhabited,  analogy  would  lead  us  to  think  that  they 
are,  and  that  the  most  distant  star  that  shines  in 
the  arch  of  heaven  may  give  light  and  heat  to  living 
beings  under  the  care  and  government  of  Him  who 
enlivens  the  densest  forest  with  the  hum  of  insects, 
and  populates  even  a  drop  of  water  with  its  teeming- 
millions  of  animalcules. 

Divisions  of  the  Planets. — The  planets  are  divided 
into  two  classes  :  (1).  Inferior,  or  those  whose  orbits 
are  within  that  of  the  earth — viz.,  Mercurj^,  Venus ; 
(2).  Superior,  or  those  whose  orbits  are  beyond  that 

*  Astronomers  conceive  the  universe  to  contain  worlds  in  every  possible  stage  of 
development,  from  the  primary,  gaseous  nebula,  to  a  worn-out,  dead  globe,  like  the 
moon.  At  a  certain  period  in  its  existence,  each  world  may  be  fitted  to  support  life. 
Millions  may  now  be  in  that  condition  ;  others  may  be  approaching,  while  others  liave 
passed  it. 


64  THE  SOLAR  SYSTEM. 

of  the  earth — viz.,  Mars,  Jupiter,  Saturn,  Uranus, 
Neptune. 

Motions  of  a  Planet  as  seen  from  the  Sun. — Could 
we  stand  at  the  sun  and  watch  the  movements  of  the 
planets,  they  would  all  be  seen  revolving  with  dif- 
ferent velocities  in  the  order  of  the  zodiacal  signs. 
But  to  us,  standing  on  one  of  the  planets,  itself  in 
motion,  the  effect  is  changed.  To  an  observer  at  the 
sun  all  the  motions  would  be  real,  while  to  us  many 
are  only  apparent.  The  position  of  a  planet,  as  seen 
from  the  center  of  the  sun,  is  called  its  heliocentric 
place  ;  as  seen  from  the  center  of  the  earth,  its  geo- 
centric place.  When  Venus  is  at  inferior  conjunc- 
tion, an  observer  at  the  sun  would  see  it  in  the  oppo- 
site part  of  the  heavens  from  that  in  which  it  would 
appear  to  him  if  viewed  from  the  earth. 

Motions  of  an  Inferior  Planet — An  inferior  planet 
is  never  seen  by  us  in  any  part  of  the  sky  opposite 
to  the  sun  at  the  time  of  observation.  It  cannot 
recede  from  him  as  much  as  90°,  or  \  the  circum- 
ference, since  it  moves  in  an  orbit  entirely  enclosed 
by  the  orbit  of  the  earth.  Twice  in  every  revolution 
it  is  in  conjunction  ( 6  )  with  the  sun, — an  inferior 
conjunction  (A)  wlien  it  comes  between  the  earth 
and  the  sun,  and  a  superior  conjunction  (B)  when 
the  sun  lies  between  it  and  the  earth. 

When  the  planet  attains  its  greatest  distance  east 
or  west  (as  we  see  it)  from  the  sun,  it  is  said  to  be  at 
its  greatest  elongation. 

When  passing  from  B  to  A  it  is  east  of  the  sun, 
and  from  A  to  B  it  is  west  of  the  sun.  When  east  of 
the  sun,  it  sets  later  than  the  sun,  and  hence  is 


THE   PLANETS. 


65 


evening  star :  when  west  of  the  sun,  it  rises  earlier 
than  the  sun,  and  hence  is  morning  star.  An  inferior 
■planet  is  never  visible  when  in  superior  conjunction, 
as  its  light  is  then  lost  in  the  greater  brilliancy  of 
the  sun.     When   in   inferior  conjunction,  it  some- 


Conjunctions  of  Inferior  Planet. 


times  passes  in  front  of  the  sun,  and  appears  to  us  as 
a  round,  black  spot  swiftly  moving  across  his  disk. 
This  is  called  a  transit. 

Retrograde  Motion  of  an  Inferior  Planet. — 
Suppose  the  earth  at  A  (Fig.  26),  and  the  planet  at  B. 


66 


THE  SOLAR  SYSTEM. 


Now,  while  the  earth  is  passing  to  F,  the  planet  will 
pass  to  D, — the  arc  AF  being  shorter  than  BD,  be- 
cause the  nearer  a  planet  is  to  the  sun  the  greater  its 
velocity.  While  the  planet  is  at  B,  we  locate  it  at 
C  on  the  ecliptic,  in  Gemini ;  but  at  D,  it  appears  to 
us  to  be  at  G,  in  Taurus.     So  that  the  planet  has 


Betroffrade  Motion. 


retrograded  through  an  entire  sign  on  the  ecliptic, 
while  its  course  all  the  while  has  been  directly  for- 
ward in  the  order  of  the  signs  ;  and  to  an  observer  at 
the  sun,  such  would  have  been  its  motion. 

Phases  of  an  Inferior  Planet. — An  inferior 
planet  presents  all  the  phases  of  the  moon.  At  supe- 
rior conjunction,  the  whole  illumined  disk  is  turned 
toward  us ;  but  the  planet  is  lost  in  the  sun's  rays  : 


THE  PLANETS.  67 

therefore  neither  Mercury  nor  Venus  ever  presents  a 
complete  circular  appearance,  like  the  full  moon.  A 
little  before  or  after  superior  conjunction,  an  inferior 
planet  may  be  seen  with  a  telescope  ;  but  the  whole 
of  the  light  side  is  not  turned,  toward  us,  and  so  the 
planet  appears  gihhous,  like  the  moon  between  the 
first  quarter  and  full.  At  its  greatest  elongation, 
the  planet  shows  us  only  one-half  its  illumined  disk  ; 
this  decreases,  becoming  more  and  more  crescent 
toward  inferior  conjunction,  at  which  time  the  un- 
illumined  side  is  toward  us. 

Fig.  27. 


Phases  of  an  Inferior  Planet. 

Motions  of  a  Superior  Planet.— The  superior  planet 
moves  in  an  orbit  which  entirely  surrounds  that  of  the 
earth.  When  the  earth  is  at  E  (Fig.  28),  the  planet 
at  L  is  said  to  be  in  opposition  to  the  sun  ( S  ).  It 
is  then  at  its  greatest  distance  from  him— 180°.  The 
planet  is  on  the  meridian  at  midnight,  while  the  sun 
is  on  the  corresponding  meridian  on  the  opposite  side 
of  the  earth  ;  or  the  planet  may  be  rising,  when  the 
sun  is  just  setting.    When  the  planet  is  at  N,  it  is  in 


68 


THE  SOLAR  SYSTEM. 


conjunction,  and  being  lost  in  the  sun's  rays  is  invis- 
ible to  us.  When  90^  east  or  west  of  the  sun,  the 
planet  is  said  to  be  in  quadrature  (□). 

Retrograde  Motion  of  a  Superior  Planet. — Sup- 
pose the  earth  to  be  at  E  and  the  planet  at  L,  and 
that  we  move  on  to  G  while  the  planet  passes  on  to 


Retrogrnde  Motion  ofaSujKrior  Planet. 


O— the  distance  EG  being  longer  than  LO,  the  re- 
verse of  what  takes  place  in  the  movements  of  the 
inferior  planets  ;  at  E,  we  should  locate  the  planet  at 
P  on  the  ecliptic,  in  the  sign  Cancer ;  but  at  G,  it 
would  appear  to  us  at  Q,  in  the  sign  Gemini,  having 


THE   PLANETS.  69 

apparently  retrograded  on  the  ecliptic  the  distance 
PQ,  while  it  was  all  the  time  moving  on  in  the 
direct  order  of  the  signs.  Now,  suppose  the  earth 
passes  on  to  I  and  the  planet  to  U,  we  should  then 
see  it  at  the  point  W,  further  on  in  the  ecliptic  than 
Q,  which  indicates  direct  motion  again,  and  at  some 
point  near  Q  the  planet  must  have  appeared  without 
motion. 

After  this,  it  will  continue  direct  until  the  earth  has 
completed  a  large  portion  of  her  orbit,  as  we  can 
easily  see  by  imagining  various  positions  of  the  earth 
and  planet,  and  then  drawing  lines  as  we  have  just 
done,  noticing  whether  they  indicate  direct  or  retro- 
grade motion.  The  greater  the  distance  of  a  planet 
the  less  it  will  retrograde,  as  we  can  perceive  by 
drawing  another  orbit  outside  the  one  represented  in 
the  cut,  and  making  the  same  suppositions  concern- 
ing it  as  those  we  have  already  explained. 

Sidereal  and  Synodic  Revolution, — The  interval 
of  time  required  by  a  planet  to  perform  a  revolution 
from  one  fixed  star  back  to  it  again,  is  termed  a 
sidereal  revolution  {sidus,  a  star). 

1,  The  interval  of  time  between  two  similar  con- 
junctions of  an  inferior  planet  with  the  earth  and 
the  sun  is  termed  a  synodic  revolution.  Were  the 
earth  at  rest,  there  would  be  no  difference  between  a 
sidereal  and  a  synodic  revolution,  and  the  planet 
would  come  into  conjunction  twice  in  each  revolu- 
tion. Since,  however,  the  earth  is  in  motion,  it  fol- 
lows that,  after  the  planet  has  completed  its  sidereal 
revolution,  it  must  overtake  the  earth  before  they 
can  both  come  again  into  the  same  position  with 


70  THE   SOLAR   SYSTEM. 

regard  to  the  sun.  The  faster  a  planet  moves,  the 
sooner  it  can  do  this.  Mercury,  traveling  at  a 
greater  speed  and  on  an  inner  orbit,  accomplishes 
it  much  more  quickly  than  Venus.  The  synodic 
period  always  exceeds  the  sidereal, 

2.  The  interval  between  two  successive  conjunc- 
tions or  oppositions  of  a  superior  planet  is  also 
termed  a  synodic  j^e volution.  Since  the  earth  moves 
so  much  faster  than  anj'  superior  planet,  it  fol- 
lows that,  after  it  has  completed  a  sidereal  revo- 
lution, it  must  overtake  the  planet  before  they  can 
again  come  into  the  same  position  with  regard  to 
the  sun.  The  slower  the  planet,  the  sooner  this 
can  be  done.  Uranus,  making  a  sidereal  revolution 
in  eighty-four  years,  can  be  overtaken  more  quickly 
than  Mars,  which  makes  one  in  less  than  two 
years.  It  consequently  requires  over  a  second 
revolution  for  the  earth  to  catch  up  with  Mars, 
only  ^  of  a  second  one  to  overtake  Jupiter,  and 
but  little  over  ^hi  of  a  second  one  to  come  up  with 
Uranus. 

Planets  as  Evening  and  Morning  Stars. — The  in- 
ferior planets  are  evening  stars  from  superior  to 
inferior  conjunction  :  and  the  superior  planets,  from 
opposition  to  conjunction.  During  the  other  part  of 
their  revolutions,  they  are  morning  stars. 


Mercury  is  evening  st 

Venus 

Mars 

Jupiter 

Saturn 

FraoQUS 


iir. , about  2  months. 


H 
13 

6 


THE   PLANETS.  71 


I.    VULCAN    (hypothetical). 

Supposed.  Discovery. — Le  Vcrrier,  having  detected  an  error  in  tho 
assumed  motion  of  Mercury,  suggested,  in  the  autumn  of  1859,  that  there 
might  be  an  interior  planet,  which  was  the  cause  of  this  disturbance.  On 
this  being  made  public,  M.  Lescarbault,  a  French  physician  and  au 
amateur  astronomer,  stated  that  on  Mai-ch  26  of  that  year  lie  had  seen  a 
dark  body  pass  across  the  sun's  disk,  which  might  have  been  the 
unknown  planet.  Le  Verrier  visited  him,  and  found  his  instruments 
rough  and  home-made,  but  singularly  accurate.  His  clock  was  a  .simple 
pendulum,  consisting  of  an  ivory  ball  hanging  from  a  nail  by  a  silk  thread. 
His  observations  were  on  prescription  paper,  covered  with  grease  and 
laudanum.  His  calculations  were  chalked  on  a  board,  which  he  planed  olf 
to  make  room  for  fresh  ones.  Le  Verrier  became  satisfied  that  a  new  planet 
had  been  discovered  by  this  enthusiastic  observer,  and  congratulated  liim 
upon  his  deserved  success. 

On  March  20,  1862,  Mr.  Lummis,  of  Manchester,  England,  noticed  a 
rapidly-moving,  dark  spot,  apparently  the  transit  of  an  inner  planet. 
During  the  total  eclipse  of  July  29,  1878,  Professor  Watson,  of  Ann  Arlior 
Observatory,  and  Dr.  Lewis  Swift,  of  Rochester,  claimed  to  liave  seen 
two  Intra-Mercurial  planets.  As  yet,  however,  the  existence  of  the  planet 
is  not  generally  conceded.  The  name  Vulcan  and  the  sign  of  a  hammer 
have  been  given  to  it.  Its  distance  from  the  .sun  has  been  estimated  at 
13,000,000  miles,   and   its  periodic  time  (its  year)  at  twenty  days. 

II.    MERCURY. 

The  fleetest  of  the  gods.     Sign,    s  ,  his  wand. 

Description. — Mercury  is  nearest  to  the  sun  of  any 
of  the  definitely-known  planets.  When  the  sky  is 
very  clear,  we  maj  sometimes  see  it,  just  after  sun- 
set, as  a  bright,  sparkling  star,  near  the  western 
jiorizpn.   Its  elevation  increase's  evening  hj  evening, 


72  THE   SOLAR   SYSTEM. 

but  never  exceeds  28°.*  If  we  watch  it  closely,  we 
shall  find  that  the  planet  again  approaches  the  sun 
and  becomes  lost  in  his  rays.  Some  days  after- 
ward, just  before  sunrise,  we  can  see  the  same  planet 
in  the  east,  rising  higher  each  morning,  until  its 
greatest  elevation  equals  that  which  it  before  at- 
tained in  the  west.  Thus  the  planet  appears  slowly 
but  steadily  to  oscillate  like  a  pendulum,  to  and  fro, 
from  one  side  to  the  other  of  the  sun.  The  ancients, 
deceived  by  this  puzzling  movement,  failed  to  dis- 
cover the  identity  of  the  two  stars,  and  called  the 
morning  star  Apollo,  the  god  of  day,  and  the  evening 
star  Mercury,  the  god  of  thieves,  who  walk  to  and 
fro  in  the  night-time  seeking  plunder,  f 

On  account  of  the  nearness  of  Mercury  to  the  sun, 
it  is  difficult  to  be  detected.  |  It  is  said  that  Coper- 
nicus, an  old  man  of  seventy,  lamented  in  his  last 
moments  that,  much  as  he  had  tried,  he  had  never 
been  able  to  see  it.  In  our  latitude  and  climate,  we. 
can  generally  easily  find  it  if  we  watch  for  it  at  the 
time  of  its  greatest  elongation,  as  commonly  given 
in  the  almanac. 

Motion  in  Space. — Mercury  revolves  around  the 
sun  at  a  mean  distance  of  about  30,000,000  miles.   Its 

*  This  distance  varies  much,  owing  to  the  eccentricity  of  Mercury's  orbit. 

t  The  Greeks  gave  to  Mercury  the  additional  name  of  "The  Sparkling  One."  The 
astrologists  looked  upon  it  as  the  malignant  jilanet.  The  chemists,  because  of  its 
extreme  swiftness,  applied  the  name  to  quicksilver.  The  most  ancient  account  that  we 
have  of  this  planet  is  given  by  Ptolemy,  in  his  Almagest ;  he  states  its  location  on  the 
15th  of  November,  205  b.  c.  The  Chinese  also  state  that  on  June  9,  llS  a.  d.,  it  was 
near  the  Beehive,  a  cluster  of  stars  in  Cancer.  Astronomers  tell  us  tliat,  according  to 
the  best  calculations,  it  was  at  that  date  witliin  less  than  1°  of  that  group. 

t  An  old  English  writer  by  the  name  of  Goad,  in  1680,  humorously  termed  this  planet, 
"  A  squinting  lacquey  of  the  sun,  who  seldom  shows  his  head  in  these  parts,  as  if  he 
were  in  debt" 


Mercury.  73 

orbit  is  the  most  eccentric  (flattened)  of  any  among 
the  eight  principal  planets,  so  that,  although  when  in 
perihelion  it  approaches  to  within  about  28,000,000 
miles,  in  aphelion  it  speeds  away  15,000,000  miles 
further,  or  to  the  distance  of  over  43,000,000  miles. 
Being  so  near  the  sun,  its  motion  in  its  orbit  is  cor- 
respondingly rapid, — viz.,  thirty  miles  per  second.* 

The  Mercurial  year  comprises  only  about  eighty- 
eight  days,  or  nearly  three  of  our  months.  Mercury 
is  thought  to  rotate  upon  its  axis  in  about  the  same 
time  as  the  earth,  so  that  the  length  of  the  Mercurial 
day  is  nearly  the  same  as  that  of  the  terrestrial 
one. 

Though  Mercury  thus  completes  a  sidereal  revolu- 
tion around  the  sun  in  eighty-eight  days,  yet  to  pass 
from  one  inferior  or  superior  conjunction  to  the  next 
(a  synodic  revolution)  requires  IIG  days.  The  reason 
of  this  is,  that  when  Mercury  comes  around  again 
to  the  point  of  its  last  conjunction,  the  earth  has 
gone  forward,  and  it  requires  twenty-eight  days  for 
the  planet  to  overtake  us. 

The  Distance  from  the  Earth  varies  still  more 
than  the  distance  from  the  sun.  At  inferior  con- 
junction, Mercury  is  between  the  earth  and  the  sun, 
and  its  distance  from  us  is  the  difference  between 
the  distance  of  the  earth  and  of  the  planet  from  the 
sun  :  at  superior  conjunction,  it  is  the  stun  of  these 
distances.  Its  apparent  diameter  in  these  different 
positions  varies  in  the  same  proportion  as  the  dis- 
tance, or  nearly  three  to  one.   The  greatest  and  least 

♦  At  this  rate  of  speed,  we  could  cross  the  Atlautic  Ocean  in  two  minutes. 


74  THE  SOLAR  SYSTEM. 

distances  vary  as  either  planet  happens  to  be  in 
aphelion  or  perihelion.* 

Dimensions. — Mercury  is  about  3,000  miles  in  di- 
ameter. Its  volume  is  about  ^V  that  of  the  earth — 
1.  e.,  it  would  require  twenty  globes  as  large  as  Mer- 
cury to  make  one  the  size  of  the  earth,  or  25,000,000 
to  equal  the  sun.  It  is  \  denser  than  the  earth,  its 
mass  is  nearly  ^V  that  of  the  earth,  and  a  stone  let 
drop  upon  its  surface  would  fall  7^  feet  the  first 
second.  Its  specific  gravity  is  not  far  from  that  of 
tin.  A  pound  weight  removed  to  Mercury  would 
weigh  only  about  seven  ounces. 

Seasons. — As  Mercury's  axis  is  much  inclined  from 
a  perpendicular  (perhaps  70°),  its  seasons  are  peculiar. 
There  are  no  distinct  frigid  zones  ;  but  large  regions 
near  the  poles  have  six  weeks  of  continuous  day  and 
torrid  heat,  alternating  with  a  night  of  equal  length 
and  arctic  cold.  The  sun  shines  perpendicularly 
upon  the  torrid  zone  only  at  the  equinoxes,  while  he 
sinks  far  toward  the  southern  horizon  at  one  solstice, 
and  as  far  toward  the  northern  horizon  at  the 
other,  t  The  equatorial  regions,  therefore,  during 
each  revolution,  are  modified  in  their  temperature 
from  torrid  to  temperate,  and  the  tropical  heat  is 
experienced  alternately  toward  the  north  and  the 
south  of  what  we  call  the  temperate  zones. 

There  is  no  marked  distinction  of  zones  as  with  us, 
but  each  zone  changes  its  character  twice  during  the 

*  If  at  inferior  conjunction  Mercury  is  in  aphelion  and  the  earth  in  perihelion,  its 
distance  from  us  is  only  91,500,000  —  43,000,000  =  48,500,000  miles.  If  at  superior  con- 
junction Mercury  is  in  ai>helion  and  the  earth  in  aphelion  also,  its  distance  from  us 
is  94,500,000  +  4:i,000,000=  137,500,000  miles. 

t  Read  a  chapter  entitled  "  The  Fiery  World,"  in  Proctor's  Poetry  of  Astronomy. 


MERCURY. 


75 


Mercurial  year,  or  eight  times  during  the  terrestrial 
one.  An  inhabitant  of  Mercury  must  be  accustomed 
to  sudden  and  violent  vicissitudes  of  temperature. 
At  one  time,  the  sun  not  only  thus  pours  down  its 
vertical  rays,  and  in  a  few  weeks  after  sinks  far 
toward  the  horizon,  but,  on  account  of  Mercury's 

Fig.  29. 


Orhit  and  Seasons  of  Mermiry. 


elliptical  orbit,  when  in  perihelion  the  planet  ap- 
proaches so  near  the  sun  that  the  heat  and  light  are 
ten  times  as  great  as  ours,  while  in  aphelion  it  re- 
cedes so  as  to  reduce  the  amount  to  four  and  a  half 
times.  The  average  heat  is  about  seven  times  that 
of  the  earth, — a  temperature  sufficient  to  turn  water 
into  steam,  and  even  to  melt  zinc. 


76  THE  SOLAR  SYSTEM. 

The  relative  length  of  the  days  and  nights  is  much 
more  variable  than  with  us.  The  sun,  apparently 
seven  times  as  large  as  it  seems  to  us,  must  be  a 
magnificent  spectacle,  and  illumine  every  object 
with  insufferable  brilliancy.  The  evening  sky  is, 
however,  lighted  by  no  moon. 

Telescopic  Features. — Through  the  telescope,  Mer- 
cury presents  all  the  phases  of  the  moon,  from  a  slen- 
der crescent  to  gibbous,  after  which  its  light  is  lost  in 
that  of  the  sun.  These  phases  prove  that  Mercury  is 
spherical,  and  shines  by  the  light  reflected  from  the 
sun.  Being  an  inferior  planet,  we  never  see  it  when 
full,  and  hence  the  brightest,  nor  when  nearest  the 
earth,  as  then  its  dark  side  is  turned  toward  us. 

t)wing  to  the  dazzling  light,  and  the  vapors  almost 
always  hanging  around  our  horizon,  this  planet  has 
not  of  late  received  much  attention  ;  the  data  here 
given  are  mainly  based  upon  the  observations  of  the 
older  astronomers,  and  are,  therefore,  not  universally 
accepted.  Mercury  is  thought  by  some  to  have  a 
dense,  cloudy  atmosphere,  that  materially  dimin- 
ishes the  intensity  of  its  heat  and,  perhaps,  makes 
it  habitable,  though  others  assert  that  the  atmos- 
phere is  too  insignificant  to  be  detected.  Some  dark 
bands  about  the  planet's  equator  indicate,  perhaps, 
an  equatorial  zone.  There  are,  also,  lofty  heights 
which  intercept  the  light  of  the  sun.  and  deep  valleys 
plunged  in  shade.  One  mountain  is  claimed  to  be 
over  eleven  miles  high,  or  about  ^  the  diameter  of 
the  planet.* 

♦  Tlie  height  of  the  loftiest  peak  of  the  Himalayas  is  only  29,000  feet,  or  about  ^Vos 
part  of  the  earth's  diameter. 


VENUS.  ?7 


III.    VENUS. 

The  Queen  of  Beauty.      Sign   ?  ,  a  looking-glass. 

Description. — Venus,  the  next  in  order  to  Mercury, 
is  the  most  brilliant  of  the  planets.*  She  presents 
the  same  appearances  as  Mercury.  Owing,  however, 
to  the  larger  size  of  her  orbit,  her  greatest  apparent 
oscillations  are  nearly  48°  east  and  west  of  the  sun,t 
or  about  20^  more  than  those  of  Mercury.  She  is 
therefore  seen  much  earlier  in  the  morning  and 
much  later  at  night.  She  is  morning  star  from  in- 
ferior to  superior  conjunction,  and  evening  star  from 
superior  to  inferior  conjunction. 

Venus  is  the  most  brilliant  about  five  weeks  before 
and  after  inferior  conjunction,  at  which  time  the 
planet  is  bright  enough  to  cast  a  shadow  at  night. 
If,  in  addition,  at  this  time  of  greatest  brilliancy, 
Venus  is  at  or  near  her  highest  north  latitude,  she 
may  be  seen  with  the  naked  eye  in  full  daylight. ;}: 
This  occurs  once  in  eight  years — the  interval  required 
for  the  earth  and  planet  to  return  to  the  same  situa- 
tion in  their  orbits  ;  eight  complete  revolutions  of  the 

*  When  visible  before  sunrise,  she  was  called  by  the  aiicieiits  Phosphorus,  Lueifer, 
or  tl;e  Morning  Star,  and  when  slie  shone  in  the  evening  after  sunset,  Hesperus,  Vesjier, 
or  the  Evening  Star. 

t  This  distance  varies  only  about  3",  owing  to  the  slight  eccentricity  of  Venus's 
orbit. 

J  Arago  relates  that  Buonaparte,  upon  repairing  to  the  Luxembourg,  when  the 
Directory  was  about  to  give  him  a  fete,  was  much  surprised  at  seeing  the  multitude 
paying  more  attention  to  the  heavens  above  the  palace  than  to  him  or  his  brilliant  staff. 
Upon  inquiry,  he  learned  that  these  curious  persons  were  observing  with  astonisliment 
a  star  which  they  supposed  to  be  that  of  the  Conqueror  of  Italy.  The  emperor  himself 
was  not  inditfiirent  when  his  piercing  eye  caught  the  clear  lustre  of  Venus  smiling  upon 
him  at  midday. 


t8  THE   SOLAli  SVSTEif. 

earth  about  the  sun  occupying  nearly  the  same  time 
as  thirteen  of  Venus. 

Motion  in  Space. — Venus  has  an  orbit  the  most 
nearly  circular  of  any  of  the  principal  planets.  Her 
mean  distance  from  the  sun  is  about  67,000,000  miles, 
which  varies  at  aphelion  and  perihelion  1,000,000 
miles, — a  contrast  to  Mercury,  which  varies  15,000,000 
miles. 

Venus  makes  a  complete  revolution  around  the 
sun  in  about  225  days,  at  the  mean  rate  of  twenty- 
two  miles  per  second ;  hence  her  year  is  equal  to 
about  seven  and  one-half  of  our  months.  This  is  a 
sidereal  revolution,  as  it  would  appear  to  an  ob- 
server at  the  sun  ;  a  sijuodic  re  »'olution  requires  584 
days. 

I\iercury,  we  remember,  catches  up  with  the  earth 
in  twenty-eignt  days  after  it  reaches  the  point  where 
it  left  the  earth  at  the  last  inferjr>r  conjunction.  But 
it  takes  Venus  nearly  two  and  a  half  revolutions  to 
overtake  the  earth  anu  eome  into  the  same  conjunc- 
tion again.  This  grows  out  of  the  fact  that  she 
has  a  longer  orbit  than  Mercury,  and  moves  only 
about  one-sixth  faster  than  the  earth,  while  Mer- 
cury travels  nearly  twice  as  fast  as  our  planet. 
Venus  rotates  upon  her  axis  in  about  twenty-four 
hours  ;  so  the  length  of  her  day  does  not  differ  essen- 
tially from  ours. 

Distance  from  the  Earth. — Like  that  of  Mercury, 
the  distance  of  Venus  from  the  earth,  when  in  in- 
ferior conjunction,  is  the  difference  between  the  dis- 
tances of  the  two  planets  from  the  sun  ;  when 
in  superior  conjunction,  the  sum  of  these  distances. 


When  nearest  to  us,  Yenus  is  only  about  25,000,000 
miles  away. 

Figure  30  represents  her  apparent  dimensions  at 
the  extreme,  mean,  and  least  distances  from  us. 
The  variation  is  nearly  as  the  numbers  10,  18,  and  65. 
It  would  be  natural  to  think  that  the  planet  is  the 
brightest  when  the  nearest,  and  thus  the  largest,  but 


Extreine,  Mean,  and  Least  Apparent  Size  of  Venus  ;  and  Iter  Piloses. 

we  should  remember  that  then  the  bright  side  is 
toward  the  sun,  and  the  unillumined  side  toward  us. 
Indeed,  at  the  period  of  greatest  brilliancy,  of  which 
we  have  spoken,  only  about  one-fourth  of  her  light  is 
visible.  At  this  time,  however,  observers  have 
noticed  the  entire  contour  of  the-  planet  to  be  of  a 
dull  gray  hue,  as  seen  in  the  cut. 

Dimensions. — Venus  is  about  7,  GOO  miles  in  diame- 
ter. The  volume  and  density  of  the  planet  are  each 
about  nine-tenths  that  of  the  earth.  A  stone  let  fall 
upon  her  surface  would  fall  fourteen  feet  in  the  first 


go 


THE  SOLAR  SYSTEM. 


second  :  a  pound  weight  removed  to  her  equator 
would  weigh  about  fourteen  ounces.  From  this  we 
see  that  the  force  of  gravity  does  not  decrease 
exactly  in  proportion  to  the  size  of  the  planet,  any 
more  than  it  increases  with  the  size  of  the  sun. 
The  reason  is,  that  the  body  is  brought  nearer  the 
mass  of  the  small  planet,  and  so  feels  its  attraction 
more  fully  than  when  far  out  upon  the  circumfer- 
ence of  a  large  body, — the  attraction  increasing  as 
the  square  of  the  distance  from  the  particles  de- 
creases. 

Seasons. — Since  the  axis  of  Venus  is  very  much 
inclined  from  a  perpendicular,  her  seasons  are  similar 

Fig.  31. 


V:  iia.<  lit  ill  I-  SUstic 


to  those  of  Mercury.  The  torrid  and  temperate  zones 
overlap  each  other,  and  the  polar  regions  have,  alter- 
nately, at  one  solstice  a  torrid  temperature,  and  at 
the  other  a  prolonged  arctic  cold.  The  inequality  of 
the  nights  is  very  marked.     The  heat  and  light  are 


VENUS. 


81 


double  that  of  the  earth,  while  the  circular  form  of 
her  orbit  gives  nearly  an  equal  length  to  her  four 
seasons. 

If  the  inclination  of  her  axis  is  75°,  as  some  as- 
tronomers hold,  her  tropics  must  be  75°  from  the 
equator,  and  her  polar  circles  75°  from  the  poles.  The 
torrid  zone  is,  therefore,  150°  in  width.  The  torrid 
and  frigid  zones  interlap  through  a  space  of  60",  mid- 
way between  the  equator  and  the  poles. 

Telescopic  Features. — Venus,  being  an  interior 
planet,  presents,  like  Mercury,  all  the  phases  of  the 
moon.  * 

She  is  thought  to  have  a  dense,  cloudy  atmosphere. 
This  was  suggested  by  the  fact  that  at  the  transit  of 


Fig.  SS. 


i|lil!l!„,|i|,|i| i' ,1  |i||[f'i;i; i 


ll'l.  Illlll .'''Il'l 


Crescent  and  S/iotj  ./  I'l 


Venus  over  the  sun  in  1761,  1769,  and  1883,  a  faint 
ring  of  light  surrounded  the  black  disk  of  the  planet. 


*  This  was  discovered  by  Galileo,  and  was  among  the  first  achievements  of  his  tele- 
scopic observations.  It  had  been  argued  against  the  Copeniican  system  that,  if  true, 
Venus  should  wax  and  wane  like  the  moon.  Indeed,  Copernicus  himself  boldly  declared 
that,  if  means  of  seeing  the  jdanets  more  distinctly  were  ever  invented,  Venus  would  be 
found  to  present  such  phases.  Galileo,  with  his  telescope,  proved  this  feet,  ^pd 
thus  vindicated  the  Copemican  theory. 


82  THE  SOLAK  SYSTEM. 

The  evidence  of  an  atmosphere,  as  well  as  of  moun- 
tains, however,  rests  upon  the  peculiar  appearance 
attending  her  crescent  shape. 

1.  The  luminous  part  does  not  end  abruptly ;  on 
the  contrary,  the  light  diminishes  gradually.  This 
diminution  can  be  explained  by  a  twilight  caused  by 
an  atmosphere  which  diffuses  the  rays  of  light  into 
regions  of  the  planet  where  the  sun  is  already  set. 
Thus,  on  Venus,  as  on  the  earth,  the  evenings  are 
lighted  by  twilight,  and  the  mornings  by  dawn. 

2.  The  edge  of  the  enlightened  portion  of  the 
planet  is  uneven  and  irregular.  This  appearance  is 
doubtless  the  effect  of  shadows  cast  by  mountains. 

Spots  have  been  noticed  on  her  disk  which  are  con- 
sidered to  be  traceable  to  clouds.  Herschel  thinks 
that  we  never  see  the  body  of  the  planet,  but  only  her 
atmosphere  loaded  with  vapors,  which  may  mitigate 
the  glare  of  the  intense  sunshine. 

Satellites. — Venus  is  not  known  to  have  any 
moon. 


IV.    THE    EARTH. 

Sign,  ^,  a  circle  with  Equator  and  Meridian. 

The  Earth  is  the  next  planet  we  meet  in  passing 
outward  from  the  sun.  To  the  beginner,  it  seems 
strange  enough  to  class  our  world  among  the  heav- 
enly bodies.  They  are  brilliant,  while  it  is  dark  and 
opaque  ;  they  appear  light  and  airy,  while  it  is  solid 
9,nd  firm ,:  we  see  in  it  no  motion,  while  they  ar^ 


THE   EARTH. 


83 


constantly  changing  their  position  ;  they  seem  mere 
points  in  the  sky,  while  it  is  vast  and  extended. 
Yet,  at  the  very  beginning,  we  are  to  consider  the 


Fig.  S3. 


The  Earth  in  Space. 


earth  as  a  planet  shining  brightly  in  the  heavens, 
^nd  appe,g,ring  to  other  worlds  as  a  planet  (ioes  to  us. 


84  THE  SOLAK   SYSTEM. 

We  are  to  learn  that  it  is  in  motion,  flying  througli 
its  orbit  with  inconceivable  velocity ;  that  it  is  not 
fixed,  but  hangs  in  space,  held  by  an  invisible 
power  of  gravitation  which  it  cannot  evade  ;  *  that 
it  is  small  and  insignificant  beside  the  mighty  globes 
that  so  gently  shine  upon  us  in  the  far-off  sky  ;  that, 
in  fact,  it  is  only  one  atom  in  a  universe  of  worlds, 
all  firm  and  solid,  and  all,  perhaps,  equally  fitted  to 
be  the  abode  of  life. 

Dimensions. — The  earth  is  not  "round  like  a  ball," 
but  flattened  at  the  poles.  Its  form  is  that  of  an 
oblate  spheroid.  Its  polar  diameter  is  about  7,899 
miles,  and  its  equatorial  about  7,925|.  The  com- 
pression is,  therefore,  26|  miles.  (See  table  in 
Appendix.)  If  we  represent  the  earth  by  a  globe 
one  yard  in  diameter,  the  polar  diameter  would  be 
one-tenth  of  an  inch  too  long.  The  circumfer- 
ence of  the  earth  is  nearly  25,000  miles.  Its  density 
is  about  5 1  times  that  of  water.  Its  weight  is 
6,069,000,000,000,000,000,000  tons. 

The  inequalities  of  the  earth's  surface,  arising 
from  valleys,  mountains,  etc.,  have  been  likened  to 
the  roughness  on  the  rind  of  an  orange.  On  a  globe 
sixteen  inches  in  diameter,  the  land,  to  be  in  pro- 
portion, should  be  represented  by  the  thinnest  writing 
paper,  the  hills  by  very  fine  grains  of  sand,  and  ele- 
vated ranges  by  thick  drawing-paper.  To  represent 
the  deepest  wells  or  mines,  a  scratch  should  be  made 
tha,t  would  be  invisible  except  with  a  glass. 

*  Were  the  sun's  attractive  force  upon  the  earth  replaced  by  the  largest  steel  tele- 
graph wire,  it  would  require  nine  wires  for  each  square  inch  of  the  sunward  side  gf  our 
plobe,  to  hold  the  earth  in  her  orbit. 


THE  EARTH.  85 

The  Rotundity  of  the  Earth  is  proved  in  various 
ways :  (1)  By  the  fact  that  vessels  have  sailed 
around  the  earth  ;  *  (2)  when  a  ship  is  coming  into 
port,  we  see  the  masts  first ;  (3)  the  shadow  of  the 
earth  on  the  moon  is  circular ;  (4)  the  polar  star 
seems  higher  in  the  heavens  as  we  pass  north  ;  and 
(5)  th^  horizon  expands  as  we  ascend  an  eminence,  f 
If  we  climb  to  the  top  of  a  hill,  we  can  see  further 
than  when  on  the  plain  at  its  foot.  Our  eyesight  is 
not  improved  ;  it  is  only  because  ordinarily  the  cur- 
vature of  the  earth  shuts  off  the  view  of  distant 
objects,  but  when  we  ascend  to  a  higher  point,  we 
can  see  further  over  the  side  of  the  earth.  The  cur- 
vature is  eight  inches  per  mile,  2^  x  8'"-  =  32  inches 
for  two  miles,  3^  x  8'"-  for  three  miles,  etc.  An 
object  of  these  respective  heights  would  be  just 
hidden  at  these  distances. 

Apparent  and  Real  Motion. — In  endeavoring  to 
understand  the  various  appearances  of  the  heavenly 
bodies,  it  is  well  to  remember  how  in  daily  life  we 


*  It  is  curious,  in  connection  with  this  well-known  fact,  to  recall  the  arguments 
urged  by  the  Spanisli  pliilosophers  against  the  reasoning  of  Columbus,  wlien  he  assured 
them  that  he  could  arrive  at  Asia  just  as  certainly  by  sailing  west  as  east.  "  How," 
they  asl<ed,  "  can  the  earth  be  round  ?  If  it  were,  then  on  the  opposite  side  the  min 
would  fall  upward,  trees  would  grow  with  their  brandies  down,  and  everything  would 
be  topsy-turvy.  Every  object  on  its  surface  would  certainly  fall  off,  and  if  a  ship  by 
sailing  west  should  get  around  there,  it  would  never  be  able  to  climb  up  the  side  of  the 
earth  and  get  back  again.    How  can  a  ship  sail  up  hill  ?  " 

t  "  The  history  of  aeronautic  adventure  affords  a  curious  illustration  of  this  same 
principle.  The  late  Mr.  Sadler,  the  celebrated  aeronaut,  ascended  on  one  occasion  in  a 
balloon  from  Dublin,  and  was  wafted  across  the  Irish  Channel,  when,  on  his  approach 
to  the  Welsh  coast,  the  balloon  descended  nearly  to  the  surface  of  the  sea.  By  this  time 
the  sun  was  set,  and  the  shades  of  evening  began  to  close  in.  He  threw  out  nearly  all 
his  ballast,  and  suddenly  sprang  upward  to  a  great  height,  and  by  so  doing  brought  his 
horizon  to  dip  below  the  sun,  producing  tlie  whole  phenomenon  of  a  western  sunrise. 
Subsequently  descending  in  Wales,  he,  of  course,  witnessed  a  second  sunset  on  the  sa:ue 
avening." 


86  THE   SOLAR   SYSTEM. 

transfer  motion.  On  the  cars,  when  in  rapid  move- 
ment, the  fences  and  the  trees  seem  to  gUde  by  us, 
■while  we  sit  still  and  see  them  pass.  On  a  bridge, 
when  we  are  at  rest,  we  watch  the  undulations  of 
the  waves,  until  at  last  we  come  to  think  that  they 
are  stationary  and  we  are  sweeping  up  the  stream. 

' '  In  the  cabin  of  a  large  Tcssel  going  smoothly  before  the  ^vind  on  still 
water,  or  drawn  along  a  canal,  not  the  smallest  indication  acquaints  us 
with  the  '  way  it  is  making. '  We  read,  sit,  walk,  as  if  we  were  on  land. 
If  we  throw  a  ball  into  the  air,  it  falls  back  into  our  hand  ;  if  we  drop  it, 
it  alights  at  our  feet.  Insects  buzz  around  us  as  in  the  free  air,  and  smoke 
ascends  in  the  same  manner  as  it  would  do  in  an  apartment  on  shore.  If, 
indeed,  we  come  on  deck,  the  case  is  in  some  respects  different ;  the  air, 
not  being  carried  along  with  us,  drifts  away  smoke  and  other  light  bodies, 
such  as  feathers  cast  upon  it,  apparently  in  the  opposite  direction  to  that 
of  the  ship's  progress  ;  but  in  reality  they  remain  at  rest,  and  we  leave 
them  behind  in  the  air.  And  what  is  the  earth  itself  but  the  good  ship  we 
are  sailing  in  tlirough  the  universe,  bound  round  the  sun  ;  and  as  we  sit 
here  in  one  of  the  '  berths,'  we  are  unconscious  of  there  being  any  '  way  ' 
at  all  upon  the  vessel.  On  deck,  too,  out  in  the  open  air,  it's  aU  the  same 
so  long  as  we  keep  our  eyes  on  the  ship  ;  but  immediately  we  look  over 
the  sides — and  the  horizon  is  but  the  '  gunwale '  of  our  vessel — we  see  the 
blue  tide  of  the  great  ocean  around  us  go  drifting  by  the  ship,  and  spark- 
ling with  its  million  stars  as  the  waters  of  the  sea  itself  sparkle  at  night 
between  the  tropics." 

Diurnal  Rotation  of  the  Earth  aroimd  its  Axis. 
— The  earth,  in  constantly  turning  from  west  to  east, 
elevates  our  hcirizon  above  the  stars  on  the  west,  and 
depresses  it  below  the  stars  on  the  east.  As  the 
horizon  appears  to  us  to  be  stationary,  we  assign  the 
motion  to  the  stars,  thinking  those  on  the  west, 
which  it  passes  over  and  hides,  to  have  sunk  below  it, 
or  set  J  and  imagining  those  on  the  east,  below  which 


THE  EARTH. 


87 


it  has  dropped,  to  have  moved  above  it,  or  risen.  So, 
also,  the  horizon  is  depressed  below  the  sun,  and  we 
call  it  sunrise  ;  it  is  elevated  above  the  sun,  and  we 
call  it  sunset. 

We  thus  see  that  the  diurnal  movement  of  the  sun 
by  day  and  the  stars  by  night  is  an  optical  illusion, 
— that  here  as  elsewhere  we  simply  transfer  motion. 
This  seems  easy  enough  for  us  to  understand  ;  but  it 
was  the  "stone  of  stumbling"  to  ancient  astrono- 
mers for  thousands  of  years.  Copernicus  himself,  it 
is  said,  first  thought  of  the  true  solution  while  riding 
on  a  vessel  and  noticing  how  he  insensibly  trans- 
ferred the  movement  of  the  ship  to  the  objects  on 
the  shore.  How  much  grander  the  beautiful  sim- 
plicity of  this  system  than  the  cumbersome  com- 
plexity of  the  old  Ptolemaic  belief  ! 

Diurnal  Motion  of  the  Sun. — The  explanation 
just  given  illustrates  the  apparent  motion  of  the  sun, 
and  the  cause  of  day  and  night.  Suppose  S  to  be  the 
sun.     The  earth,  E,  turning  upon  its  axis  EF  from 

Fig.  Sit. 


Daily  Motion  of  the  Sun  (Hind). 

west  to  east,  has  only  half  its  surface  illuminated  at 
one  time  by  the  sun.  To  a  person  at  D,  the  sun  is  in 
the  horizon  and  day  commences,  that  luminary  ap- 


88  THE  SOLAR  SYSTEM. 

pearing  to  rise  higher  and  higher,  with  a  westerly 
motion,  as  the  observer  is  carried  forward  easterly 
by  the  earth's  diurnal  rotation  to  A,  where  he  has 
the  sun  in  his  meridian,  and  it  is  consequently  noon. 
The  sun  then  begins  to  decline  in  the  sky  until  the 
spectator  arrives  at  B.  where  it  sets,  or  is  again  in 
the  horizon  on  the  west  side,  and  night  begins.  He 
moves  on  to  C,  which  marks  his  position  at  mid- 
night, the  sun  being  then  on  the  meridian  of  places 
on  the  opposite  part  of  the  earth,  and  he  is  brought 
round  again  to  D,  the  point  of  sunrise,  when  another 
day  commences. 

Unequal  Rate  of  Diurnal  Motion. — Different 
points  upon  the  surface  of  the  earth  revolve  with 
different  velocities.  At  the  poles  the  speed  of  rota- 
tion is  nothing,  while  at  the  equator  it  is  greatest,  or 
over  1,000  miles  per  hour.  At  Quito,  the  circle  of 
latitude  is  much  longer  than  the  one  at  the  mouth  of 
the  St.  Lawrence,  and  the  velocities  vary  in  the 
same  proportion.  The  former  place  moves  at  the 
rate  of  about  1,038  miles  per  hour :  the  latter,  682 
miles.  In  our  latitude  (41°)  the  speed  is  about  780 
miles  per  hour.  We  do  not  perceive  this  wonderful 
velocity  with  which  we  are  flying  through  the  ether, 
because  the  atmosphere  moves  with  us.* 

Were  the  earth  suddenly  to  stop  its  rotation,  the 
terrible  shock  would,  without    doubt,   destroy  the 

*  "  An  ingenious  inventor  once  suggested  that  we  should  utilize  the  earth's  rota- 
tion, as  the  most  simple  and  economical,  as  well  as  rapid  mode  of  locomotion  that  could 
be  conceived.  This  was  to  be  accomplished  by  rising  in  a  balloon  to  a  height  inacces- 
sible to  aerial  currents.  The  balloon,  remaining  immovable  in  this  calm  region,  would 
simply  await  the  moment  when  the  earth,  rotating  underneath,  should  present  the  place 
of  destination  to  the  eyes  of  travelers  who  would  then  descend.  A  well-regulated 
watch  and  an  exact  knowledge  of  longitude  would  thus  render  traveling  jiossible  fh^m 


THE    EAETH.  89 

entire  race  of  man ;  while  we,  with  houses,  trees, 
rocks,  and  even  the  oceans,  would  be  hurled,  in  one 
confused  mass,  headlong  into  space.  On  the  other 
hand,  were  the  rate  of  rotation  to  increase,  the 
length  of  the  day  would  be  proportionately  shortened, 
and  the  weight  of  all  bodies  decreased  by  the  centrif- 
ugal force  thus  produced.  If  the  rotary  movement 
should  become  swift  enough  to  reduce  the  day  to 
eighty-four  minutes,  the  force  of  gravity  would  be 
overcome,  and,  at  the  equator,  all  bodies  would  be 
without  weight;  if  the  speed  were  still  further  in- 
creased, loose  bodies  would  fly  off  from  the  earth 
like  water  from  a  swiftly-turned  grindstone,  while 
we  should  be  compelled  constantly  to  "hold  on" 
to  avoid  sharing  the  same  fate.*  But  against  such 
a  catastrophe  we  are  assured  by  the  immutability 
of  God's  laws.  "  He  is  the  same  yesterday,  to-day,  / 
and  forever." 

Unequal  Diurnal  Orbits  of  the  Stars. — In 
figure  35,  let  O  represent  our  position  on  the  earth's 
surface  ;  E  Z  B,  our  meridian  ;  E  I  B  K,  our  horizon  ; 
P  and  P',  the  north  and  south  poles  ;  Z,  the  zenith  ; 


east  to  west,  all  voyages  north  or  south  being  interdicted.  Tliis  suggestion  has  only  one 
fault ;  it  supposes  that  the  atmospherio  strata  do  not  revolve  with  the  earth.  Upon 
tliat  hypothesis,  since  we  rotate  (at  London)  with  the  velocity  of  8:33  yards  in  a 
second,  there  would  result  a  wind  in  the  contrary  direction  ten  times  more  violent  than 
the  most  terrible  hurricane.  Is  not  the  absence  of  such  a  state  of  things  a  convincing 
proof  of  the  participation  of  the  atmospheric  envelope  in  the  general  movement?" — 

GUILLEMIN. 

*  Laplace  concluded  in  1709  that  the  inequalities  of  the  earth's  rotation  were  too 
insignificant  for  measurement.  But,  more  recently.  Delaunay  has  shown  from  the  moon's 
acceleration  that  a  minute  change,  caused  by  the  friction  of  the  sea  and  atmosphere 
upon  the  earth's  surface,  has  taken  place,  producing  a  variation  in  the  length  of  the  day. 

The  acceleration  of  the  moon  in  its  path,  is,  however,  only  seven  feet  per  century, 
or  less  than  an  inch  per  annum,  and  the  time  of  the  earth's  rotation  has  increased  but 
xHeo  of  ^  second  in  2,400  years.— B.ill. 


90 


THE   SOLAR  SYSTEM. 


Z  .  the  nadir  :  aud  G I  C  K  the  celestial  equator.  Now 
P  B,  it  will  be  seen,  is  the  elevation  of  the  north  pole 
above  the  horizon,  or  the  latitude  of  the  place. 


Suppose  we  should  see  a  star  at  A,  on  the  meridian 
below  the  pole.  The  earth  revolves  in  the  direction 
G  I  C  ;  the  star  will  therefore  move  along  A  L  to  Z, 
when  it  is  on  the  meridian  above  the  pole.  It  con- 
tinues its  course  along  the  dotted  line  around  to  A 
again,  when  it  is  on  the  meridian  below  the  pole, 
having  made  a  complete  circuit  around  the  pole,  but 
not  having  descended  below  our  horizon. 

A  star  rising  at  B  would  just  touch  the  horizon  ; 
one  at  I  would  move  on  the  celestial  equator,  and 
would  be  above  the  horizon  as  long  a  time  as  it  is 
below, — twelve  hours  in  each  case  :  a  star  rising  at  M 
would  come  just  above  the  horizon  and  set  again  at  N. 


I'HE  EARTH.  i)i. 

Unequal  Diurnal  Velocities  of  the  Stars. — 
The  stars  appear  to  us  to  be  set  in  a  concave  shell 
which  revolves  daily  about  the  earth.  As  differ- 
ent parts  of  the  earth  really  rotate  with  varying  ve- 
locities, so  the  stars  appear  to  revolve  at  different 
rates  of  speed.  Those  near  the  pole,  having  a  small 
orbit,  revolve  very  slowly,  while  those  near  the 
celestial  equator  move  at  the  greatest  speed. 

Appearance  of  the  Stars  at  Different  Places 
ON  the  Earth. — Were  we  placed  at  the  north  pole, 
Polaris  would  be  directly  overhead,  and  the  stars 
would  seem  to  pass  around  us  in  circles  parallel  to 
the  horizon,  and  increasing  in  diameter  from  the 
upper  to  the  lower  ones.  Were  we  placed  at  the 
equator,  the  pole-star  would  be  at  the  horizon,  and 
the  stars  would  move  in  circles  perpendicular  to  the 
horizon,  and  decreasing  in  diameter,  north  and  south 
from  those  in  the  zenith,  while  we  could  see  one 
half  of  the  path  of  each  star.  Were  we  placed  in  the 
southern  hemisphere,  the  circumpolar  stars  would 
revolve  about  the  south  pole,  and  the  others  in 
circles  resembling  those  in  our  sky,  only  the  points  of 
direction  would  be  reversed  to  correspond  with  the 
pole.  Were  we  placed  at  the  south  pole,  the  appear- 
ance would  be  the  same  as  at  the  north  pole,  except 
that  no  star  is  there  to  mark  the  direction  of  the 
earth's  axis. 

Motion  of  the  Earth  in  Space  about  the  Sun. — 
The  earth  revolves  in  an  elliptical  path  about  the 
sun  at  a  mean  distance  of  93,000,000  miles. 

The  eccentricity  of  this  path,  which  is  greater 
than  that  of  the    orbit  of    Venus,   changes   about 


92  THE    SOLAR  SYSTEM. 

To^FFo  per  century.  The  orbit  would,  therefore, 
finally  become  circular,  were  it  not  that,  after  the 
lapse  of  some  thousands  of  years,  the  eccentricity 
will  begin  to  increase  again,  and  will  thus  vary 
through  all  time  within  definite,  although  yet  un- 
determined limits.  The  circumference  is  nearl}" 
600,000,000  miles,  and  the  earth  pursues  this  wonder- 
ful journey  at  the  rate  of  over  eighteen  miles  per 
second. 

This  revolution  of  the  earth  about  the  sun  gives 
rise  to  various  phenomena,  of  which  we  shall  now 
proceed  to  speak. 

1.  Change  in  the  Appearance  of  the  Heavens 
IN  Different  Months. — In  Fig.  36,  suppose  A  B  C  D 
to  be  the  orbit  of  the  earth,  and  E  F  G  H  the 
sphere  of  the  fixed  stars,  surrounding  the  sun  in 
every  direction.  When  our  globe  is  at  A,  the  stars 
about  E  are  on  the  meridian  at  midnight.  Being 
seen  from  the  earth  in  the  quarter  opposite  to 
the  sun,  they  are  favorably  placed  for  observa- 
tion. The  stars  at  G,  on  the  contrary,  will  be  in- 
visible, for  the  sun  intervenes  between  them  and 
the  earth  :  they  are  on  the  meridian  of  the  spec- 
tator about  the  same  time  as  the  sun,  and  are  hidden 
in  his  rays. 

In  three  months,  the  earth  has  passed  over  one- 
fourth  of  its  orbit,  and  has  arrived  at  B.  Stars 
about  F  now  appear  on  the  meridian  at  midnight ; 
those  at  E,  which  previously  occupied  their  places, 
have  descended  toward  the  west  ;  while  those  about 
G  are  just  coming  into  sight  in  the  east. 

In  three  months  more,  the  earth  is  situated  at  C, 


THE  EARTH.  93 

and  stars  about  G  shine  in  the  midnight  sky,  those 
at  F  having,  in  their  turn,  vanished  in  the  west ; 

Fig.  36. 

H 

^       *■ 


*        * 


* 


•■*■* 


X* 


*  * 


v 

Appearance  of  the  Heavens  in  Different  Seasons  (Hind). 

stars  at  E  are  on  the  meridian  at  noon,  and  conse- 
quently hidden  in  daylight ;  and  those  about  H  are 
just  making  their  appearance  in  the  east.  One  revo- 
lution of  the  earth  will  bring  the  same  stars  again 
on  the  meridian  at  midnight. 

Thus  the  earth's  motion  round  the  sun  as  a  center 
explains  the  varied  aspect  of  the  heavens  in  the 
summer  and  winter  skies. 


94:  THE  SOLAR  SYStEYt. 

2.  Yearly  Path  of  the  Sun  Through  the 
Heavens. — We  have  spoken  of  the  diurnal  motion 
of  the  sun.  We  shall  now  speak  of  its  second  ap- 
parent motion,  its  yearly  path  among  the  stars, — the 
ecliptic*  If  we  look  at  Fig.  37,  we  can  see  how  the 
motion  of  the  earth  in  its  orbit  is  transferred  to  the 
sun,  and  causes  him  to  appear  to  travel  in  a  fixed 
path  through  the  heavens.  When  the  earth  is  in  any 
part  of  its  orbit,  the  sun  seems  to  us  to  be  in  the 
point  directly  opposite.  For  example,  when  the 
earth  is  in  Libra  (=^)t — autumnal  equinox — the  sun 
is  in  Aries  (T) — vernal  equinox;  when  the  sun 
enters  the  next  sign,  Taurus  (d),  the  earth  has 
passed  on  to  Scorpio  (m).  Thus,  as  the  earth  moves 
through  her  orbit,  the  sun  seems  to  pass  along  the 
opposite  side  of  the  ecliptic,  making  the  circuit  of 
the  heavens  in  a  year,  and  returning,  at  the  end  of 
that  time,  to  the  same  place  among  the  stars.  The 
ecliptic  crosses  the  celestial  equator  at  two  points, 
called  the  equinoxes.     (See  page  30). 

*  Tliis  yearly  movement  of  the  sun  among  the  fixed  stars  is  not  so  apparent  to  us  as 
his  daily  motion,  because  his  superior  light  blots  out  the  stars.  But  if  we  notice  a  star 
at  the  western  horizon  just  at  sunset,  we  can  tell  what  constellation  the  sun  is  in  :  then 
wait  two  or  three  nights,  and  we  shall  find  that  this  star  has  set,  and  others  have  taken 
its  jilace.  Thus  we  can  trace  the  sun  through  the  year  in  his  path  among  the  fixed 
stars  in  the  horizon. 

t  When  we  say  "  the  earth  is  in  Libra,"  we  mean  that  a  spectator  placed  at  the  sun 
would  see  the  earth  in  that  part  of  the  heavens  which  is  occupied  by  the  sign  Libra, 
while  a  spectator  on  the  earth  would  see  the  sun,  at  the  same  time,  in  that  part  of  the 
heavens  which  is  occupied  by  the  sign  Aries.  Just  so,  on  June  21st,  the  earth  enters 
Capricorn,  and  the  sun,  Cancer.  It  is  customary,  however,  having  reference  solely  to 
the  sun's  place,  to  locate  the  vernal  equinox  in  Aries,  and  the  autumnal  equinox  in 
Libra ;  the  summer  solstice  in  Cancer,  and  the  winter  solstice  in  Capricorn.  In  figure 
37,  the  terms  "summer  solstice,"  "autumnal  equinox,"  etc.,  refer  to  the  season 
upon  the  earth,  and  to  the  location  of  the  sun  in  the  ecliptic,  but  are  not  the  names  of 
those  points  on  tlie  earth's  orbit.  The  zodiacal  signs  are  inserted  for  convenience  of 
illustration,  to  show  where  the  earth  would  be  located  by  a  solar  spectator  :  the  pupil 
should  remember,  however,  that  the  signs  belong  to  the  ecliptic— which  is  the  projec- 
tion of  the  plane  of  the  earth's  orbit  upon  the  celestial  sphere,  and  not  to  the  earth's  path. 


the  earth.  95 

3.  Apparent  Movement  of  the  Sun,  North  and 
HouTH. — Having  now  spoken  of  the  apparent  diurnal 
and  annual  motions  of  the  sun,  tliere  yet  remains  a 
third  motion.  In  summer,  at  midday,  the  sun  is 
high  in  the  heavens  ;  in  winter,  he  is  low,  near  the 
southern  horizon.  In  summer,  he  is  a  long  time  above 
the  horizon  ;  in  winter,  a  short  time.  In  summer,  he 
rises  and  sets  north  of  the  east  and  west  points ;  in 
winter,  south  of  the  east  and  west  points.  This 
subject  is  so  intimately  connected  with  the  next, 
that  we  shall  understand  it  best  when  taken  in  con- 
nection with  that  topic. 

4.  Change  of  the  Seasons.  Variation  in  Length 
OP  Day  and  Night.— By  studying  Fig.  37,  and 
imagining  the  various  positions  of  the  earth  in 
its  orbit,  let  us  try  to  understand  the  following 
points  : 

I.  Obliquity  of  the  ecliptic. — The  axis  of  the  earth 
is  inclined  23^°  from  a  perpendicular  to  its  orbit. 
This  angle  is  called  the  obliquity  of  the  ecliptic. 

II.  Parallelism  of  the  axis. — In  all  parts  of  the 
orbit,  the  axis  of  the  earth  is  parallel  to  itself,  and 
points  almost  exactly  toward  the  North  Star  (p.  217). 

Nature  reveals  to  us  nothing  more  permanent  than 
the  axis  of  rotation  in  anything  that  is  rapidly 
turned.  It  is  its  rotation  that  keeps  a  boy's  hoop 
from  falling.  For  the  same  reason,  a  quoit  retains 
its  direction  when  whirled,  and  stays  in  the  same 
plane  at  whatever  angle  it  may  be  thrown.  A  man 
slating  a  roof  wishes  to  throw  a  slate  to  the  ground  ; 
he  whirls  it  perpendicularly,  and  it  will  strike  on  the 
edge  without  breaking.     So  long  as  a  top  spins  there 


Ihe  Urbit  of  the  Earth  as  seen  by  an  Observer  at  the  Sun.    (See  Note,  p.  dU-) 


THE   EARTH.  9? 

is  no  danger  of  its  falling,  since  its  tendency  to  keep 
its  axis  of  rotation  parallel  is  greater  than  the  attrac- 
tion of  the  earth.  This  wonderful  law  would  lead 
us  to  think  that  the  axis  of  the  earth  always  points 
in  the  same  direction,  even  if  we  did  not  know  it 
from  direct  observation, 

III.  The  rays  of  the  sun  strike  the  various  portions 
of  the  earth,  when  in  any  position,  at  different 
angles. — When  the  earth  is  in  Libra,  and  also  when 
in  Aries,  the  sun's  rays  strike  vertically  at  the  equa- 
tor, and  more  and  more  obliquely  in  the  northern 
and  southern  hemispheres,  as  the  distance  from  the 
equator  increases,  until  at  the  poles  they  strike 
almost  horizontally. 

This  variation  in  the  direction  of  the  rays  pro- 
duces a  corresponding  variation  in  the  intensity  of 
the  sun's  heat  and  light  at  different  places,  and 
accounts  for  the  difference  between  the  torrid  and 
polar  regions. 

IV.  As  the  earth  changes  its  position  the  angle  at 
which  the  rays  strike  any  portion  is  varied. — Take 
the  earth  when  it  is  in  Capricornus  (V5)  and  the  sun 
in  Cancer  (s).  He  is  now  overhead,  23|°  iiorth  of 
the  equator.  His  rays  strike  less  obliquely  in  the 
northern  hemisphere  than  when  the  earth  was  in 
Libra,  Let  six  months  elapse  :  the  earth  is  now  in 
Cancer  and  the  sun  in  Capricornus  ;  and  he  is  over- 
head, 23J°  south  of  the  equator.  His  rays  strike  less 
obliquely  in  the  southern  hemisphere  than  before, 
but  in  the  northern  hemisphere  more  obliquely. 
These  six  months  have  changed  the  direction  of  the 
sun's  rays  on  every  part  of  the  earth's  surface.     This 


98  THE  SOLAR  SYSTEM. 

accounts  for  the  difference  in  temperature  between 
summer  and  winter.* 

\V.  Equinoxes. — At  the  equinoxes,  one  half  of  each 
hemispht3re  is  illuminated  :  hence  the  name  Equinox 
{cequtis,  equal ;  and  7iox,  night).  At  these  points  of 
the  orbit,  the  days  and  nights  are  equal  over  the 
entire  earth, f  each  being  twelve  hours  in  length. 

VI.  Xorthern  and  soiitheiii  hemispheres  unequally 
illuminated. — While  one-half  of  the  earth  is  con- 
stantly illuminated,  the  proportion  of  the  northern 
or  the  southern  hemisphere  that  is  in  daylight  or 
darkness  varies  at  all  times,  except  at  the  equinoxes. 
When  more  than  half  of  a  hemisphere  is  in  the  light, 
its  days  are  longer  than  the  nights,  and  vice  versa. 

VII.  The  seasons  and  the  comparative  length  of 
the  days  and  nights  in  the  South  Temperate  Zone,  at 
any  time,  are  the  reverse  of  those  in  the  North  Tem- 
perate Zone,  except  at  the  Equinoxes,  ichen  the  days 
and  nights  are  of  equal  length. 

VIII.  Tlie  Summer  Solstice. — At  the  time  of  the 
summer  solstice,  which  occurs  about  the  21st  of 
June,  the  sun  is  overhead  23V"  north  of  the  equator, 
and  if  his  vertical  rays  could  leave  a  golden  line  on 
the  surface  of  the  earth  as  it  rotates,  they  would 
mark  the  Tropic  of  Cancer,  The  sun  is  at  its  fur- 
thest northern  declination  ;  he  ascends  the  highest 
he  is  ever  seen  above  our  horizon,  and  rises  and  sets 
north  of  the  east  and  west  points.  He  seems  now 
to  stand  still  in  his  northern  and  southern  course. 


♦  The  long  nights  and  short  days  of  winter,  and  the  sliort  nights  and  long  days  of 
summer,  are  also  important  factors  in  producing  this  difference  of  temperature, 
t  Except  a  small  space  at  each  pole. 


THE  EARTH.  ^  99 

and  hence  the  name  Solstice  {sol,  the  sun  ;  sto,  I 
stand).  The  days  in  the  north  temperate  zone  are 
longer  than  the  nights.  It  is  our  summer,  and  the 
21st  of  June  is  the  longest  day  of  the  year. 

In  the  south  temperate  zone  it  is  winter,  and  the 
shortest  day  of  the  year.  The  circle  that  separates 
day  from  night  extends  23|°  beyond  the  north  pole, 
and  if  the  sun's  rays  could  in  like  manner  leave  a 
golden  line  on  that  day,  they  would  trace  on  the 
earth  the  Arctic  Circle.  It  is  the  noon  of  the  long 
six-months  polar  day.  The  reverse  is  true  at  the 
Antarctic  Circle,  and  it  is  there  the  midnight  of  the 
long  six-months  polar  night  (p.  117). 

IX.  The  Autumnal  Equinox. — The  earth  crosses 
the  aphelion  point  about  the  1st  of  July.  It  is  then 
at  its  furthest  distance  from  the  sun,  which  each 
day  rises  and  sets  a  trifle  further  toward  the  south, 
passing  through  a  lower  circuit  in  the  heavens.  At 
the  time  of  the  autumnal  equinox,*  the  22nd  of  Sep- 
tember, he  is  on  the  equinoctial,  and  if  his  vertical 
rays  could  leave  a  line  of  golden  light,  they  would 
mark  on  the  earth  the  circle  of  the  equator.  It  is 
autumn  in  the  north  temperate  zone  and  spring  in 
the  south  temperate  zone.  The  days  and  nights 
are  equal  over  the  whole  earth,  the  sun  rising  at 
6  A.M.  and  setting  at  G  p.m.,  exactly  in  the  east  and 
the  west,  where  the  equinoctial  intersects  the  horizon. 

X.  TJie  Winter  Solstice. — The  sun  after  passing 
the  equinoctial — "crossing  the  line" — sinks  lower 
toward  the   southern    horizon    each    day.     At  the 

*  The  precise  time  of  the  equinoxes  and  solstices  varies  each  year,  but  wtthin  a  small 
limit. 


100  THE  SOLAR  SYSTEM. 

time  of  the  winter  solstice,  about  the  21st  of  De- 
cember, the  sun  is  directly  overhead  23|^°  south  of 
the  equator,  and  if  his  vertical  rays  could  leave  a 
line  of  golden  light,  they  would  mark  on  the  earth's 
surface  the  Tropic  of  Capricorn.  He  is  at  his  furthest 
southern  declination,  and  rises  and  sets  south  of  the 
east  and  west  points.  It  is  our  winter,  and  the  21st 
of  December  is  the  shortest  day  of  the  year. 

In  the  south  temperate  zone  it  is  summer,  and 
the  longest  day  of  the  year.  The  circle  that  separates 
day  from  night  extends  23^°  beyond  the  south  pole, 
and  if  the  sun's  rays  in  like  manner  could  leave  a 
line  of  golden  light,  they  would  mark  the  Antarctic 
Circle.  It  is  there  the  noon  of  the  long  six-months 
polar  day.  At  the  Arctic  Circle  the  reverse  is  true  ; 
the  rays  fall  23|^°  short  of  the  north  pole,  and  it  is 
there  the  midnight  of  the  long  six-months  polar 
night.  Here  again  the  sun  appears  to  us  to  stand 
still  a  day  or  two  before  retracing  his  course,  and  it 
is  therefore  called  the  Winter  Solstice. 

XI,  The  Vernal  Equinox. — The  earth  reaches  its 
perihelion  about  the  31st  of  December.  It  is  then 
nearest  the  sun,  which  rises  and  sets  each  day  fur- 
ther and  further  north,  and  climbs  up  higher  in  the 
heavens  at  midday.  Our  days  gradually  increase  in 
length,  and  our  nights  shorten  in  the  same  propor- 
tion. About  the  21st  of  March  the  sun  reaches  the 
equinoctial,  at  the  vernal  equinox.  He  is  overhead 
at  the  equator,  and  the  days  and  nights  are  again 
equal.  It  is  our  spring,  but  in  the  south  temperate 
zone  it  is  autumn. 

XII.  Yearly  path  finished. — The  earth  moves  on  in 


AT  LOS  ANGELES 

THE  EARTH.  101 

its  orbit  through  the  spring  and  the  summer  months. 
The  sun  continues  his  northerly  course,  ascending 
each  day  higher  in  the  heavens,  and  his  rays  becom- 
ing less  and  less  oblique.  About  the  21st  of  June,  he 
again  reaches  his  furthest  northern  declination,  and 
is  at  the  summer  solstice. 

We  have  thus  traced  the  yearly  path,  and  noticed 
the  course  of  the  changing  seasons,  with  the  length 
of  the  days  and  nights.  The  same  series  has  been 
repeated  through  the  ages  of  the  past,  and  will  be 
through  the  future  till  time  shall  be  no  more. 

XIII.  Distance  of  the  earth  from  the  sun  varies. — 
We  notice,  from  what  we  have  just  seen,  that  we  are 
nearer  the  sun  in  winter  than  in  summer  by  3,000,000 
miles.  The  obliqueness  with  which  the  rays  strike 
the  north  temperate  zone  at  that  time  prevents  our 
receiving  any  special  benefit  from  this  favorable 
position  of  the  earth. 

XIV.  Southern  summer.  —  The  inhabitants  of  the 
south  temperate  zone  have  their  summer  while  the 
earth  is  in  perihelion,  and  the  sun's  rays  are  about 
^V  warmer  than  when  in  aphelion,  our  summer-time. 
This  will  perhaps  partly  account  for  the  extreme 
heat  of  their  season.*  The  southern  winters,  for  a 
similar  reason,  are  colder  ;  and  this  makes  the  aver- 
age yearly  temperature  about  the  same  as  ours. 

XV.  Extremes  of  heat  and  cold  not  at  the  solstices. 
— We  do  not  have  our  greatest  heat  at  the  time  of  the 
summer  solstice,  nor  our  greatest  cold  at  the  winter 

*  Captain  Sturt,  in  speaking  of  tlie  extreme  heat  of  Australia,  says  tliat  niatclies  acci- 
dentally dropped  on  the  ground  were  ignited.  A  recent  official  rejicrt  states  that,  in 
South  Australia,  January.  188:1,  the  heat,  in  the  sun,  was  180°— only  32°  below  the 
boiling-point- 


102  THE  SOLAR  SYSTEM. 

solstice.  After  the  21st  of  June,  the  earth,  already 
warmed  by  the  genial  spring  days,  continues  to 
receive  more  heat  from  the  sun  by  day  than  it  radi- 
ates by  night :  thus  its  temperature  still  increases. 
On  the  other  hand,  after  the  21st  of  December,  the 
earth  continues  to  become  colder,  because  it  loses 
more  heat  during  the  night  than  it  receives  during 
the  day. 

XVI.  Summer  longer  than  icinter. — As  the  sun  is 
not  in  the  center  of  the  earth's  orbit,  but  at  one  of 
its  foci,  the  earth,  from  the  time  of  the  vernal  to  that 
of  the  autumnal  equinox,  passes  through  more  than 
one-half  of  its  orbit.  The  summer  is,  therefore,  longer 
than  the  winter.  The  difference  is  enhanced  by  the 
variation  in  the  earth's  velocity  at  aphelion  and  at 
perihelion. 

XVII.  Varying  velocity  of  earth. — From  the  time 
of  the  vernal  equinox  until  the  earth  passes  its 
aphelion,  the  solar  attraction  tends  to  check  its 
speed  ;  thence  until  the  time  of  the  autumnal  equinox, 
the  attraction  is  partly  in  the  direction  of  its  motion, 
and  so  increases  its  velocity.  The  same  principle 
applies  when  going  to  and  from  perihelion. 

XYIII.  Curious  appearance  of  the  smi  at  the  north  jxilc. — "  To  a  person 
standing  at  the  north  pole,  the  sun  appears  to  sweep  horizontally  around 
the  sky  every  twenty-four  hours,  witliout  any  perceptible  variation  in  its 
distance  from  the  horizon.  It  is,  however,  slowly  lising,  until,  on  the  21st 
of  June,  it  is  t\\'enty-three  degrees  and  twenty-eight  minutes  above  the 
horizon,  a  little  more  than  one-fourth  of  the  distance  to  the  zenith.  This 
is  the  highest  point  it  ever  reaches.  From  this  altitude,  it  slowly  descends, 
its  track  being  represented  by  a  spiral  or  screw  with  a  very  fine  thread  ; 
and  in  the  course  of  three  months  it  worms  its  way  down  to  the  horizon, 
which  it  reaches  on  the  22nd  of  September.     On  this  day  it  slowly  sweej^ 


THE  EARTH.  103 

around  the  sky,  with  its  face  half  hidden  below  the  icy  sea.  It  still  con- 
tinues to  descend,  and  after  it  has  entirely  disappeared  it  is  still  so  near  the 
horizon  that  it  carries  a  bright  twilight  around  the  heavens  in  its  daily 
circuit.  As  the  sun  sinks  lower  and  lower,  this  twilight  grows  gradually 
fainter,  till  it  fades  away.  December  21st,  the  sun  is  23^°  below  the 
horizon,  and  this  is  the  midnight  of  the  dark  polar  winter.  From  this 
date,  the  sun  begins  to  ascend,  and  after  a  time  it  is  heralded  by  a  faint 
dawn,  which  circles  slowly  around  tlie  horizon,  completing  its  circuit  every 
twenty-four  hours.  This  dawn  grows  gradually  brighter,  and  on  the  22nd 
of  March  the  peaks  of  ice  are  gilded  with  the  first  level  rays  of  the  six- 
months  day.  The  bringer  of  this  long  day  continues  to  wind  his  spiral 
way  upward,  till  he  reaches  his  highest  place  on  the  21st  of  June,  and  his 
annual  course  is  completed." 

XIX.  Results,  if  the  axis  of  the  earth  were  perpen- 
dicular to  the  ecliptic. — The  sun  would  then  always 
appear  to  move  through  the  equinoctial.  He  would 
rise  and  set  every  day  at  the  same  points  on  the 
horizon,  and  pass  through  the  same  circle  in  the 
heavens,  while  the  days  and  nights  would  be  equal 
the  year  round.  There  Avould  be  near  the  equator  a 
fierce  torrid  heat,  while  north  and  south  the  climate 
would  change  into  temperate  spring,  and,  lastly, 
into  the  rigors  of  a  perpetual  winter. 

XX.  Results,  if  the  equator  of  the  earth  ivere  per- 
pendicular to  the  ecliptic. — Were  this  the  case,  to 
a  spectator  at  the  equator,  as  the  sun  leaves  the 
vernal  equinox,  he  would  each  day  pass  through 
a  smaller  circle,  until  at  the  summer  solstice  he 
would  reach  the  north  pole,  when  he  would  halt  for 
a  time,  and  then  slowly  return  in  an  inverse  man- 
ner. 

In  our  own  latitude,  the  sun  would  make  his 
diurnal    rotations  as    described,   his    rays    shining 


104  THE  SOLAR  SYSTEM. 

past  the  north  pole  further  and  further,  until  we 
were  included  in  the  region  of  perpetual  day,  when 
he  would  seem  to  wind  in  a  spiral  course  up  to  the 
north  pole,  and  then  return  in  a  descending  curve  to 
the  equator. 

Precession  of  the  Equinoxes. — We  have  spoken 
of  the  equinoxes  as  if  they  were  stationary.  Over 
two  thousand  years  ago,  Hipparchus  (see  page  8) 
found  that  they  are  slowly  falling  back  along  the 
ecliptic.  Modern  astronomers  fix  the  rate  at  about 
50"  of  space  annually.  If  we  mark  either  point  in 
the  ecliptic  where  the  days  and  nights  are  equal 
over  the  earth — at  which  time  the  plane  of  the 
earth's  equator  passes  exactly  through  the  center  of 
the  sun— we  shall  find  the  sun  comes  back  to  that  po- 
sition the  next  year,  50"  (50  m.  20  s.  of  time)  earlier. 
This  remarkable  effect  is  called  the  Precession  of  the 
Equinoxes,  because  the  position  of  the  equinoxes  in 
any  year  precedes  that  which  they  occupied  the  year 
before.  Since  the  circle  of  the  ecliptic  is  divided 
into  360^,  it  follows  that  the  time  occupied  by  the 
equinoctial  points  in  making  a  complete  revolution 
at  the  rate  of  50".2  per  year  is  25,817  years. 

Results  of  the  Precession  of  the  Equinoxes. — 
In  Fig.  37,  we  see  that  the  plane  of  the  earth's 
equator  is  inclined  to  that  of  the  ecliptic.  In  order 
that  the  plane  of  the  terrestrial  equator  should  pass 
through  the  sun's  center  50"  earlier,  it  is  necessary 
that  the  plane  itself  should  slightly  change  its  place. 
The  axis  of  the  earth  is  always  perpendicular  to 
this  plane,  hence  it  follows  that  the  axis  is  not  rigor- 
ously parallel  to  itself.    It  varies  in  direction^  so  that; 


'itt&  fiABT H. 


105 


the  north  pole  describes  a  small  circle  in  the  starry 
vault  twice  23|^°  in  diameter. 

To  illustrate  this,  let  us  suppose  that,  after  a  series 
of  years,  the  position  of  the  earth's  equator  has 
changed  from  efh  to  g  Kl  (Fig.  38).  The  inclina- 
tion of  the  axis  of  the  earth,  CP,  to  CQ,  the  pole  of 
the  ecliptic,  remains  unchanged  ;  but  as  it  must  kirn 


Change  of  Earth's  Eqitator  and  Axis.* 

with  the  equator,  its  position  is  moved  from  CP  to 
CP',  and  the  pole  of  the  earth  slowly  traces  the  por- 
tion of  a  circle,  PP'.  The  direction  of  this  motion  is 
the  same  as  that  of  the  hands  of  a  watch,  or  the  re- 
verse of  the  revolution  of  the  earth.  The  position  of 
the  north  pole  in  the  heavens  is  gradually  but  almost 
insensibly  changing.  It  is  now  distant  from  the  north 
polar  star  about  1\°.    It  will  continue  to  approach  it 

*  See  in  the  Appendix  a  description  of  a  simple  apparatus  for  illustrating  this 
subject 


lot)  THE  SOLAR  SYST.EM. 

until  they  are  not  more  than  half  a  degree  apart. 
In  12,000  years,  Lyra  will  be  our  polar  star  :  4,500 
years  ago  the  polar  star  was  the  bright  star  Alpha 
in  the  constellation  Draco.     (See  p.  217). 

As  the  right  ascension  of  the  stars  is  reckoned 
eastward  from  the  vernal  equinox  along  the  equi- 
noctial, the  precession  of  the  equinoxes  increases  the 
R.  A.  of  the  stars  50"  per  year.  On  this  account,  star 
maps  should  be  accompanied  by  the  date  of  their 
calculation,  that  they  may  be  corrected  to  corre- 
spond Avith  this  annual  variation. 

The  constellations  of  the  zodiac  (see  p.  31)  are 
fixed  in  the  heavens,  while  the  signs  are  simply 
abstract  divisions  which  move  with  the  equinox. 
When  named,  the  sun  was  in  both  the  sign  and 
the  constellation  Aries,  at  the  time  of  the  vernal 
equinox ;  but  since  then  the  equinoxes  liave  retro- 
graded nearly  a  whole  sign,  so  that  now,  while  the 
vernal  equinox  is  in  the»sign  Aries,  this  sign  cor- 
responds to  the  constellation  Pisces,  which  is  there- 
fore the  first  constellation  in  the  zodiac  (Fig.  86). 

Causes  of  the  Precession  of  the  Equinoxes.— 
Before  commencing  the  explanation  of  this  phenom- 
enon, it  is  necessary  to  impress  upon  the  mind  a  few 
facts.  (1.)  The  earth  is  n^)t  a  perfect  sphere,  but  is 
swollen  at  the  equator.  It  is  like  a  sphere  covered 
with  padding,  increasing  in  thickness  from  the  poles 
to  the  equator  ;  this  gives  it  a  turnip-like  shape. 
(2.)  The  attraction  of  the  sun  is  greater  the  nearer 
a  body  is  to  it.  (3.)  The  attraction  is  not  for  the 
earth  as  a  mass,  but  for  each  particle  separately. 

In  the  figure,  the  position  of  the  earth  at  the  time 


The  earth. 


1U7 


of  the  winter  solstice  is  represented.  P  is  the  north 
pole  ;  a  b,  the  plane  of  the  ecliptic  ;  C,  the  center  of 
the  earth  ;  C  Q,  a  line  perpendicular  to  the  ecliptic  ; 
the  angle  Q  C  P,  the  obliquity  of  the  ecliptic.  In  this 
position,  the  equatorial  padding  of  which  we  have 
spoken — the  ring  of  matter  about  the  equator — is  not 
turned  exactly  toward  the  sun,  but  is  elevated  above 
it.  Now  the  attraction  of  the  sun  pulls  the  part  D 
more  strongly  than  the  center ;  the  tendency  of  this 

Fig.  S9. 


hijlit^nce  oftlie  Sun  oh  a  Mountain  near  the  Equator. 


is  to  bring  D  down  to  a,  and  to  lift  I  toward  b.  The 
attraction  for  C  is  greater  than  for  I,  so  it  tends  to 
draw  C  away  from  I,  and,  as  at  the  same  time  D 
tends  toward  a,  to  pull  I  up  toward  b.  The  effect 
of  this,  one  would  think,  would  be  to  change  the 
inclination  of  the  axis  C  P  toward  C  Q,  and  make  it 
more  nearly  perpendicular  to  the  ecliptic.  This 
would  be  the  result  if  the  earth  were  not  rotating 
upon  its  axis. 

Let  us  consider  the  case  of  a  mountain  near  the 
equator.  This,  if  the  sun  did  not  act  upon  it,  would 
pass  through  the  curve  H  D  E  in  the  course  of  a 
semi-rotation  of  the  earth.     But,  it  is  nearer  the  sun 


108  THE  SOLAR  SYSTEM. 

than  is  the  center  C ;  the  attraction  therefore  tends 
to  pull  the  mountain  downward  and  tilt  the  earth 
over,  as  we  have  just  described ;  so  the  mountain 
will  pass  through  the  curve  H/g,  and,  instead  of 
crossing  the  ecliptic  at  E,  will  cross  at  g,  a  little 
sooner  than  it  otherwise  would.  The  same  influence, 
though  in  a  less  degree,  obtains  on  the  opposite  side 
of  the  earth.  The  mountain  passes  around  the  earth 
in  a  curve  nearer  to  h,  and  crosses  the  ecliptic  a 
little  earlier. 

The  same  reasoning  will  apply  to  each  mountain 
and  to  all  the  jjrotuberant  mass  near  the  equatorial 
regions.  The  final  effect  is  slightly  to  turn  the 
earth's  equator  so  that  it  intersects  the  ecliptic 
sooner  than  it  would  were,  it  not  for  this  attraction. 
At  the  summer  solstice,  the  same  tilting  motion  is 
produced.  At  the  equinoxes,  the  plane  of  the  earth's 
equator  passes  through  the  center  of  the  sun,  and 
therefore  there  is  no  tendency  to  change  of  position. 
As  the  axis  C  P  must  move  with  the  equator,  it 
slowly  revolves,  keeping  its  inclination  unchanged, 
around  C  Q,  the  pole  of  the  ecliptic,  describing,  in 
about  2G,000  years,  a  small  circle  twice  23^°  in  diam- 
eter. 

Precession  illustrated  in  the  spinning  of  a  top. 
— This  motion  of  the  earth's  axis  is  singularly  illus- 
trated in  the  spinning  of  a  top,  and  the  more  so 
because  the  forces  are  of  an  opposite  character  to 
those  which  act  on  the  earth,  and  thus  produce  an 
opposite  effect.  We  have  seen  that,  if  the  earth  had 
no  rotation,  the  sun's  attraction  on  the  "padding" 
at  the  equator  would  bring  C  P  nearer  to  C  Q,  but 


THE  EAflTH. 


109 


Fig.  UO. 


that,  in  consequence  of  this  rotation,  the  effect 
really  produced,  is  that  C  P,  the  earth's  axis,  slowly 
revolves  around  C  Q,  the  pole  of  the  heavens,  in  a 
direction  opposite  to  that  of  rotation. 

In  Fig.  40,  let  C  P  be  the  axis  of  a  spinning  top, 
and  C  Q  the  vertical  line. 
The  direct  tendency  of 
the  earth's  attraction  is 
to  bring  C  P  further  from 
C  Q  (or  to  make  the  top 
fall),  and  if  the  top  were 
not  spinning  this  would 
be  the  result ;  but,  in  con- 
sequence of  the  rotary 
motion,  the  inclination 
does  not  sensibly  alter 
(until  the  spinning  is  re- 
tarded by  friction),  and  so  C  P  slowly  revolves 
around  C  Q  in  the  same  direction  as  that  of  rotation. 
-^Nutation  {nutatio,  a  nodding). — We  have  noticed 
the  sun  as  producing  precession ;  the  moon  has, 
however,  treble  his  influence  ;  for  although  her  mass 
is  not  ^-^.ooV.o-ro  part  that  of  the  sun,  yet  she  is  400 
times  nearer  and  her  effect  correspondingly  greater.* 
The  moon's  orbit  does  not  lie  parallel  to  the  ecliptic, 
but  is  inclined  to  it.  Now  the  sun  attracts  the  moon, 
and  disturbs  its  path,  as  he  would  that  of  the  moun- 
tain we  have  supposed,  and  the  effect  is  the  same. 
The  intersections  of  the  moon's  orbit  with  the  ecliptic 
travel  backward,  completing  a  revolution  in  about 
18  years. 


spinning  of  a  To^). 


See  the  Differential  effect  of  the  Sun  and  the  Moon,  under  the  head  of  the  Tides. 


110  'IHE   SOLAR  SYSTEM. 

During  half  of  this  time,  the  moon's  orbit  is  in- 
clined to  the  ecliptic  in  the  same  way  as  the  earth's 
equator ;  during  the  other  half,  it  is  inclined  in  the 
opposite  way.  In  the  former  state,  the  moon's  at- 
tractive tendency  to  tilt  the  earth  is  very  small,  and 
the  precession  is  slow  ;  in  the  latter,  the  tendency  is 
great,  and  precession  goes  on 
..■^^^\:7<^  rapidly.   The  consequence  of  this 

//  ^x)         is,  that  the  pole  of  the  earth  is 

\  ^      irregularly    shifted,    so    that    it 

'^  \     travels  in  a  slightly  curved  line, 

/  .        0      giving  it  a  kind  of  "wabbling" 

^  rr       ox  "nodding"  motion,  as  shown 

^''\:;yr:::^.../^  — though  greatly  exaggerated — 

Path  of  the  North  Pole  in  the      in  Fig.  41.     The  obliquity  of  the 

Heavens.  ,.     ,.  -1*1  •  t  ,-,^^0 

ecliptic,  which  we  consider  23| 
(23°  27'  15",  Jan.  1,  1884.     See  p.  29),  is  the  mean  of 
the  irregularly  curved  line  and  is  represented  by 
the  dotted  circle. 

Periodical  change  in  the  obliquity  of  the 
ECLIPTIC. — Although  it  is  sufficiently  near  for  all 
general  purposes  to  consider  the  obliquity  of  the 
ecliptic  invariable,  yet  this  is  not  strictly  the  case. 
It  is  subject  to  a  small  but  appreciable  variation  of 
about  46"  per  century.  This  is  caused  by  a  slow 
change  of  the  position  of  the  earth's  orbit,  due  to  the 
attraction  of  the  planets.  The  effect  of  this  move- 
ment is  gradually  to  diminish  the  inclination  of  the 
earth's  equator  to  the  ecliptic  (the  obliquity  of  the 
ecliptic).  This  will  continue  for  a  time,  when  the 
angle  will  as  gradually  increase ;  the  extreme  limit 
of  change  being  only  1°  21'.     The  orbit  of  the  earth 


THE  EARTfl.  ,       111 

thus  vibrates  backward  and  forward,  each  oscillation 
requiring  a  period  of  10,000  years. 

The  change  is  so  intimately  blended,  in  its  effect 
upon  the  obliquity  of  the  ecliptic,  with  that  caused 
by  precession  and  nutation,  that  they  are  separable 
only  in  theory  ;  in  fact,  they  all  combine  to  produce 
the  waving  motion  we  have  already  described.  As 
a  consequence  of  this  variation  in  the  obliquity  of 
the  ecliptic,  the  sun  does  not  now  come  so  far  north 
nor  decline  so  far  south  as  formerly ;  while  the 
position  of  all  the  terrestrial  circles — the  Troj)ics  of 
Cancer,  Capricorn,  etc. — is  constantly  but  slowly 
changing.  As  the  result  of  this  variation  in  the 
position  of  the  orbit,  some  stars  which  were  once 
just  south  of  the  ecliptic  are  now  north  of  it,  and 
others  that  were  just  north  are  now  a  little  further 
north ;  thus  the  latitude  of  these  stars  is  gradually 
changing. 

Change  in  the  major  axis  (line  of  apsides)  of 
THE  earth's  orbit. — Besides  all  the  changes  in  the 
position  of  the  earth  in  its  orbit  due  to  precession, 
etc.,  the  line  connecting  the  aphelion  and  peri- 
helion points  of  the  orbit  itself  is  slowly  revolving. 
The  consequence  of  this  is  a  variation  in  the  length 
of  the  seasons  at  different  periods  of  time. 

In  the  year  3958  b.c,  the  earth  was  in  perihelion  at  the  time  of  the  au- 
tumnal equinox,  so  that  the  summer  and  autumn  seasons  were  of  equal 
length,  but  shorter  than  the  winter  and  spring  seasons,  which  were  also  equal.  * 

*  Tliere  is  much  (liscreiiancy  in  the  views  held  concerning  tlie  Great  Year  of  llie 
a.stnmomers,  as  it  is  often  called.  (See  Steele's  Geology,  pp.  272-3,  note.)  The  state- 
ment made  in  the  text  is  that  held  by  Loekyer,  Hind,  and  others.  The  dates  are  those 
given  hy  Chambers  in  his  Descriptive  Astronomy  (3rd  Edition),  where  the  subject  is  fully 
described. 


112  THE  SOLaH  system. 

In  the  year  1267  A.D.,  the  earth  was  in  j^erihelion  at  the  time  o^ 
the  winter  solstice,  December  21,  instead  of  January  1st,  as  now  ;  the 
spring  quarter  was  therefore  equal  to  the  summer  one,  and  the  autumn 
quarter  to  the  winter  one,  the  former  being  the  longer.  In  the  year  6493 
A.  D. ,  the  earth  will  be  in  perihelion  at  the  time  of  the  vernal  equinox  ; 
summer  will  then  be  equal  to  autumn  and  winter  to  spring,  the  former 
seasons  being  the  longer.  In  the  year  11719  A.D.,  the  earth  will  be  in 
perihelion  at  the  time  of  the  summer  solstice  :  finally,  in  16945  A.D.,  the 
cycle  will  be  completed.and  the  autumnal  equinox  will  again  coincide  with 
the  earth's  perihelion. 

Permanence  in  the  Midst  of  Change. — We  thus 
see  that  the  ecliptic  is  constantly  modifying  its  ellip- 
tical shape  ;  that  the  orbit  of  the  earth  slowly  oscil- 
lates upward  and  downward  :  that  the  north  pole 
steadily  turns  its  long  index-finger  over  a  dial  that 
marks  26.000  years ;  that  the  earth,  accurately 
poised  in  space,  gently  nods  and  bows  to  the  attrac- 
tion of  sun,  moon,  and  planets.  *  Thus  changes  are 
taking  place  that  would  ultimately  entirely  reverse 
the  order  of  nature.  But  each  of  these  variations 
has  its  bounds,  beyond  which  it  cannot  pass.  The 
promise  made  to  man  is  that,  ''while  the  earth  re- 
maineth,  seed-time  and  harvest,  and  cold  and  heat, 
and  summer  and  winter,  and  day  and  night  shall 
not  cease."  The  modern  discoveries  of  astronomy 
prove  conclusively  that  the  seasons  are  to  be  perma- 
nent ;  that  the  Creator,  amid  all  these  transitions, 
has  ordained  the  means  of  carrying  out  His  promise 
through  all  time. 

Refraction. — The  atmosphere    extends  above  the 

♦  These  oscillations  extend  throughout  the  whole  planetary  system,  the  j^erioda 
varjing  from  50,000  to  2,000,000  years.  "  Great  clocks  of  eternity,  which  beat  ages  as 
ours  beat  seconds." — Kewcomb's  Astronomy,  page  95. 


THE  EAKTH. 


113 


earth  about  500  miles   (Physics,   p.  IIG).     Near  the 
surface  it  is  dense,  while  in  the  upper  regions  it  is 


Befraction. 

exceedingly  rare.  The  rays  of  light  from  the  heav- 
enly bodies  passing  through  these  different  layers 
are  turned  downward  toward  a  perpendicular  more 
and  more  as  the  density  increases.  According  to  a 
well-known  law  of  optics  (Physics,  p.  150),  if  the  ray 
of  light  from  a  star  were  bent  in  fifty  directions 
before  entering  the  eye,  the  star  would  nevertheless 
appear  to  be  in  the  line  of  the  one  nearest  the  eye. 
The  effect  of  this  is,  that  the  apparent  place  of  a 
heavenly  body  is  higher  than  the  true  place.  The 
sun  at  S  (Fig.  42),  were  it  not  for  the  atmosphere, 
would  send  a  direct  ray  to  L.  Instead,  the  ray  at 
A  is  refracted  downward,  and  would  then  enter  the 
eye  at  N  ;  passing,  however,  through  a  layer  of  a  (iif- 


114  THE   SOLAR  SYSTEM. 

ferent  density  at  B,  it  is  again  bent,  and  meets  the 
eye  of  the  observer  at  C.  He  sees  the  sun,  not  in  the 
direction  of  the  curved  line  C  B  A  S,  but  in  that  of 
the  straight  line  C  B  SI 

The  amount  of  refraction  varies  with  the  tempera- 
ture, moisture,  and  other  conditions  of  the  atmos- 
phere. It  is  zero  for  a  body  in  the  zenith,  and  in- 
creases gradually  toward  the  horizon  (as  the  thick- 
ness of  the  intervening  atmosphere  increases),  where 
it  is  sometimes  as  much  as  35'. 

Fi'j.  hi- 


Change  of  Place  and  Appearance  of  the  Sun 
AND. THE  Moon. — The  sun  may  be  really  below  the 
horizon,  and  yet  seem  to  be  above  it.  For  example, 
on  April  20,  1837,  the  moon  was  eclipsed  before  the 
sun  had  set.  The  mean  diameter  of  both  the  sun 
and  the  moon  being  about  half  a  degree,  it  follows 
that  when  we  see  the  lower  edge  of  either  of  these 
luminaries  apparently  just  touching  the  horizon,  in 


THE   EARTH. 


lU 


reality  the  whole  disk  is  beloiv  it,  and  would  be 
hidden  were  it  not  for  the  refraction.  The  day  is 
consequently  materially  lengthened. 


Fig.iU. 


Dejonnation  qf  tlie,  Sun  near  tJie  Horizon. 

The  sun  and  the  moon  often  ai^^^ear  flattened  when 
near  the  horizon.  The  rays  from  the  lower  edge 
pass  through  a  denser  layer  of  the  atmosphere,  and 
are  therefore  refracted  more  than  those  from  the 
upper  edge  :  the  effect  of  this  is  to  make  the  vertical 
diameter  appear  less  than  the  horizontal,  and  so  to 
distort  the  figure  of  the  disk  into  an  oval  shape. 

The  dim  and  hazy  appearance  of  the  heavenly 
bodies  when  near  the  horizon  is  caused  not  only  by 
the  rays  of  light  having  to  pass  a  greater  distance 
through  the  atmosphere,  but  also  by  their  traversing 
the  denser  part.  The  intensity  of  the  solar  light  is 
so  greatly  diminished  by  going  through  the  lower 
strata,  that  we  are  then  enabled  to  look  upon  the 
s\in  without  being  dazzled  by  his  brilliant  beams. 


116  THE  SOLAR  SYSTEM. 

Twilight. — The  glow  of  light  after  sunset  and 
before  sunrise,  which  we  term  twilight,  is  caused  by 
the  refraction  and  the  reflection  of  the  sun's  rays  by 
the  atmosphere.  For  a  time  after  the  sun  has  really 
set,  the  refracted  rays  continue  to  reach  the  earth ; 
but  when  these  have  ceased,  he  still  illuminates  the 
clouds  and  upper  strata  of  the  air,  just  as  he  may 
be  seen  shining  on  the  summits  of  lofty  moun- 
tains long  after  he  has  disappeared  from  the  view  of 
the  inhabitants  of  the  plains  below.  The  air  and 
clouds  thus  illuminated  reflect  back  a  part  of  the 
light  to  the  earth.  As  the  sun  sinks  lower,  less  light 
reaches  us,  until  reflection  ceases  and  night  ensues. 
The  same  thing  occurs  before  sunrise,  only  in 
reverse  order. 

Twilight  is  usually  reckoned  to  last  until  the  de- 
pression of  the  sun  below  the  horizon  amounts 
to  18° ;  this,  however,  varies  with  the  latitude,* 
seasons,  and  condition  of  the  atmosphere.  In  the 
latitude  of  New  York,  twilight  lasts  from  1|  to  2 
hours,  the  shortest  twilight  being  in  winter,  and  the 
longest  in  summer.  Strictly  speaking,  in  the  latitude 
of  Greenwich  there  is  no  true  night  for  a  month 
before  and  after  the  summer  solstice,  but  constant 
twilight  from  sunset  to  sunrise.  The  sun  is  then 
near  the  Tropic  of  Cancer,  and  does  not  descend  so 
much  as  18°  below  the  horizon  during  the  entire 
nigi>t.  At  the  equator  the  length  of  the  evening 
twilight  is  about  1^  hours,  and  remains  almost  con- 


*  When  the  sun's  patli  is  very  oblique  to  the  horizon,  a  longer  time  is  required  for 
the  sun  to  descend  or  ascend  the  requisite  vertical  distance  of  IS"  from  the  horizon  j 
and  a  shorter  time,  when  his  path  is  niore  nearly  perpendicular. 


THE   EARTH.  117 

stant  the  entire  year.  The  twilight  is  longest  toward 
the  poles,  where  the  night  of  six  months  is  shortened 
by  an  evening  twilight  of  about  fifty  days  and  a 
morning  one  of  equal  length. 

Diffused  Light, — The  diffused  light  of  day  is  pro- 
duced in  the  same  manner  as  that  of  twilight.  The 
atmosphere  reflects  and  scatters  the  sunlight  in 
every  direction.  Were  it  not  for  this,  no  object 
would  be  visible  to  us  out  of  direct  sunshine  ;  every 
shadow  of  a  passing  cloud  would  be  pitchy  dark- 
ness ;  the  stars  would  be  visible  all  day  ;  no  window 
would  admit  light  except  as  the  sun  shone  directly 
through  it,  and  a  man  would  require  a  lantern  to  go 
around  his  house  at  noon. 

The  blue  light  reflected  to  our  eyes  from  the  at- 
mosphere above  us,  or,  more  correctly,  from  the 
vapor  in  the  air,  produces  the  optical  illusion  we 
call  the  sky.  Were  it  not  for  this,  every  time  we 
cast  our  eyes  upward  we  should  feel  like  one  gazing 
over  a  dizzy  precipice  ;  while  now  the  crystal  dome 
of  blue  smiles  down  upon  us  so  lovingly  and  beauti- 
fully that  we  call  it  heaven. 

Aberration  of  Light. — We  have  seen  that  the 
places  of  the  heavenly  bodies  are  apparently  changed 
by  refraction.  Besides  this,  there  is  another  change 
due  to  the  motion  of  light  combined  with  the  motion 
of  the  earth  in  its  orbit.  For  example  :  the  mean 
distance  of  the  earth  from  the  sun  is  about  93,000,000 
miles,  and  since  light  travels  a  little  over  186,000 
miles  per  second,  it  follows  that  the  time  occupied 
by  a  ray  of  light  in  reaching  us  from  the  sun  is 
about   8^   min.    (8  min.    18   sec.)  ;   so   that,  in   fact, 


118 


THE  SOLAK  SYSTEM. 


(1),  we  do  not  see  the  sun  as  it  is,  but  as  it  was  8| 
minutes  ago.  And  since,  during  this  time,  the  earth 
has  moved  in  its  orbit  about  20|"  (2),  we  do  not  see 
that  luminary  in  the  exact  place  it  occupies  at  the 
time  of  observation. 

Illustration. — Suppose  a  ball  let  fall  from  a  point 
P,  above  the  horizontal  line  A  B,  and  a  tube,  of 
which  A  is  the  lower  extremity,  placed  to  receive  it. 


AberrcUion  of  Light. 

If  the  tube  were  fixed,  the  ball  would  strike  it  on  the 
lower  side  ;  but  if  the  tube  were  carried  forward  in 
the  direction  A  B,  with  a  velocity  properly  adjusted 
at  every  instant  to  that  of  the  ball,  while  preserving 
its  inclination  to  the  horizon,  so  that  when  the  ball, 
in  its  natural  descent,  reached  B,  the  tube  would 
have  been  carried  into  the  position  B  Q,  it  is  evident 
that  the  ball  throughout  its  whole  descent  would  be 
found  in  the  tube ;  and  a  spectator  referring  to  the 


THE   EARTH.  119 

tube  the  motion  of  the  ball,  and  carried  along  with 
the  former,  unconscious  of  its  motion,  would  think 
that  the  ball  had  been  moving  in  an  inclined  direc- 
tion, and  had  come  from  Q. 

A  very  common  illustration  may  be  seen  almost 
any  rainy  day.  Choose  a  time  when  the  air  is  quiet, 
and  the  drops  large.  Then,  if  you  stand  still,  you 
will  see  that  the  drops  fall  vertically ;  but  if  you 
walk  forward,  you  will  see  the  drops  fall  as  if  they 
were  meeting  you.  If,  however,  you  walk  backward, 
you  will  observe  that  the  drops  fall  as  if  they  were 
coming  from  behind  you.  We  thus  see  that  the 
drops  have  an  apparent  as  well  as  a  real  motion. 

The  general  effect  of  aberration  is  to  cause 
each  star  apparently  to  describe  in  the  course  of  a 
year  a  minute  ellipse,  the  central  point  of  which  is 
the  place  thes  star  would  actually  occupy  were  our 
globe  at  rest. 

Parallax  is  the  difference  in  the  direction  of  an  ob- 
ject as  seen  from  two  different  places.  For  a  simple 
illustration,  hold  your  finger  before  you  in  front  of 
the  window.  Upon  looking  at  it  with  the  left  eye 
only,  you  will  locate  your  finger  at  some  point  on 
the  window  ;  on  looking  with  the  right  eye  only,  you 
will  locate  it  at  an  entirely  different  point.  Use  your 
eyes  alternately  and  quickly,  and  you  will  be  aston- 
ished to  see  how  your  finger  will  seem  to  change  its 
place.  Now,  the  difference  in  the  direction  of  your 
finger  as  seen  from  the  two  eyes  is  its  parallax. 

In  astronomical  calculations,  the  position  of  a 
body  as  seen  from  the  earth's  surface  is  called  its 
apparent  place,  while  that  in  which  it  would  be  seen 


130 


THE  SOLAR  SYSTEM. 


from  tke  center  of  the  earth  is  called  its  true 
place.  Thus,  in  Fig.  46,  a  star  is  seen  by  the  ob- 
server at  0  in  the  direction  OP;  if  it  could  be 
viewed  from  the  center  R,  its  direction  would  be 
in  the  line  RQ.  It  is  therefore  seen  from  O  at  a 
point  in  the  heavens  below  its  position  in  reference 
to  R.    From  looking  at  the  cut,  we  can  see  (1)^  that 


Parallax. 


the  parallax  of  a  star  near  the  horizon  is  greatest, 
while  it  decreases  gradually  until  it  disappears  alto- 
gether at  the  zenith,  since  an  observer  at  O,  as  well 
as  one  at  R,  would  see  the  star  Z  directly  overhead ; 
and  (2),  that  the  nearer  a  body  is  to  the  earth  the 
greater  its  parallax  becomes. 

It  has  been  agreed  by  astronomers,  for  the  sake  of 
imiformity,  to  correct  all  observations  so  as  to  refer 


THE  EARTH.  131 

them  to  their  true  places  as  seen  from  the  center  of 
the  earth.  Tables  of  parallax  are  constructed  for 
this  purpose.  The  question  of  parallax  is  also  of 
great  importance,  because  as  soon  as  the  parallax  of 
a  body  is  accurately  known,  its  distance,  diameter, 
etc.,  can  be  determined.  (See  Celestial  Measure- 
ments.) 

Horizontal  Parallax  is  the  parallax  of  a  body 
when  at  the  horizon.  It  is,  in  fact,  the  ea?'th's  semi- 
diameter  as  seen  from  the  body.  In  Fig.  4G,  the 
parallax  of  the  star  S  is  the  angle  O  S  R,  which  is 
measured  by  the  line  O  R — the  semi-diameter  of  the 
earth.  The  su7i's  horizontal  parallax  is  the  angle 
subtended  (measured)  by  the  earth's  semi-diameter 
as  seen  from  that  luminary.  As  the  moon  is  nearest 
the  earth,  its  horizontal  parallax  is  greater  than  that 
of  any  other  heavenly  body. 

Annual  Parallax. — The  fixed  stars  are  so  dis- 
tant from  the  earth  that  they  exhibit  no  change  of 
place  when  seen  from  different  parts  of  the  earth. 
The  lines  O  S  and  R  S  are  so  long  that  they  are 
apparently  parallel.  Astronomers,  therefore,  instead 
of  taking  the  earth's  semi-diameter,  or  4,000  miles, 
as  the  measuring  tape,  observe  the  position  of  the 
fixed  stars  at  opposite  points  in  the  earth's  orbit. ' 
This  gives  a  change  in  place  of  186,000,000  miles. 
The  variation  of  position  which  the  stars  undergo  at 
these  remote  points  is  called  their  annual  parallax. 


122 


THE  SOLAR  SYSTEM. 


THE    MOON. 

New  Moon,  (9.     First  Quarter,  «.     Full  Moon,  ®.     Last  Quarter,  #. 

Motion  in  Space. — The  orbit  of  the  moon,  consid- 
ering the  earth  as  fixed,  is  an  ellipse  of  which  our 
planet  occupies  one  of  the  foci.     Her  distance  from 


J'ath  of  the  Moon. 


the  earth,  therefore,  varies  incessantly.  At  perigee 
(peri,  near  ;  ge,  the  earth),  she  is  26,000  miles  nearer 
than  in  apogee  {ajm,  from  ;  ge,  the  earth) :  the  mean 
distance  is  about  239,000  miles.     To  reach  the  moon. 


THE  MOON.  123 

would  require  a  chain  of  thirty  globes  equal  in  size 
to  the  earth.  An  ordinary  express-train  would  take 
about  a  year  to  accomplish  the  journey. 

The  moon  completes  her  revolution  {sidereal) 
around  the  earth  in  about  27|  days ;  but,  as  the 
earth  is  constantly  passing  on  in  its  orbit  around 
the  sun,  it  requires  over  two  days  longer  before  the 
moon  comes  into  the  same  position  with  respect  to 
the  sun  and  the  earth,  thus  completing  a  synodic 
revolution,  or  lunar  month  (29^  days). 

The  real  path  of  the  moon  is  the  result  of  her 
own  motion  and  the  onward  movement  of  the  earth. 
The  two  combined  produce  a  wave-like  curve  that 
crosses  the  earth's  path  twice  each  month  ;  this, 
owing  to  its  small  diameter  compared  with  that  of 
the  earth's  orbit,  is  always  concave  toward  the  sun. 
As  the  moon  constantly  keeps  the  same  side  turned 
toward  us,  it  follows  that  she  must  rotate  on  her 
axis  once  each  month. 

Dimensions. — The  moon's  diameter  is  about  2,160 
miles.  To  equal  the  earth,  would  require  fifty  globes 
the  size  of  the  moon.  The  apparent  size  varies  with 
the  distance  ;  the  mean  is,  however,  about  one-half 
a  degree,  nearly  the  same  as  that  of  the  sun.  The 
moon  always  appears  larger  than  she  really  is,  on 
account  of  her  brightness.  This  is  the  effect  of  what 
is  termed  in  optics  Irradiation.'^  For  the  same 
reason  it  is  often  noticed  that  the  crescent  moon 
seems  to  be  a  part  of  a  larger  circle  than  the  rest  of 


*  To  illustrate  this  principle,  cut  two  circular  pieces  of  the  same  size,  one  of  black 
and  the  other  of  white  paper.  The  white  circle,  wheu  held  in  a  bright  light,  will  appear 
much  larger  than  the  black  cue. 


124  tfiE  SOLAR  SYSTEM. 

the  moon.  The  moon  appears  larger  on  the  horizon 
than  when  high  in  the  sky.  This,  however,  is  a 
mere  illusion.  *  By  an  examination  of  the  cut,  it  is 
easily  seen  that  the  moon  is  4,000  miles  nearer  when 
on  the  zenith  than  when  at  the  horizon. 

Besides  these  general  variations  in  size,  the  moon 
varies  in  apparent  size  to  different  observers.    Much 

Fig.  i8. 


The  Distance  of  the  Moon  at  fhe  Horizon  and  at  the  Zenith. 

amusement  may  be  had  in  a  large  party  or  class  by 
a  comparison  of  her  apparent  magnitude.  The  esti- 
mates will  differ  from  a  small  saucer  to  a  wash-tub. 

Librations  {lihrans,  swinging). — Though  the  moon 
presents  the  same  hemisphere  to  us,  there  are  three 
causes  which  enable  us  to  see,  in  all,  about  7^(10  of 
her  entire  surface. 

1.  The  axis  of  the  moon  is  inclined  a  little  to  her 
orbit,  as  also  her  orbit  is  inclined  to  the  earth's  orbit ; 
so,  when  her  north  pole  leans  alternately  toward  and 

•  At  the  horizon  we  compare  her  with  various  terrestrial  objects  which  lie  between  her 
and  us,  while  aloft  we  have  no  association  to  guide  us  in  judging  of  her  distance,  and  we 
are  led  to  underrate  her  size.  If  we  look  at  her  when  near  the  horizon,  through  a  roll  of 
paper,  or  the  hands  held  tube-wise,  this  illusion  will  vanish. 


fiHE  MOON.  125 

from  the  earth,  we  see  sometimes  past  her  north  and 
sometimes  past  her  south  pole.  This  is  called  lihra- 
tion  in  latitude. 

2.  The  moon's  rotation  on  her  axis  is  always  per- 
formed in  the  same  time,  while  her  movement  along 
her  orbit  is  variable ;  hence  we  occasionally  see  a 
little  further  around  each  limh  (outer  edge)  than  at 
other  times.     This  is  called  libratioti  in  longitude. 

3.  The  size  of  the  earth  is  so  much  greater  than 
that  of  the  moon,  that  an  observer,  by  the  rotation 
of  the  earth,  or  by  going  north  or  south,  can  see  fur- 
ther around  the  limbs. 

Light  and  Heat. — If  the  whole  sky  were  covered 
with  full  moons,  they  would  scarcely  make  daylight, 
since  the  brilliancy  of  the  moon  does  not  exceed 
¥00^0-00  that  of  the  sun.  That  portion  of  the  moon's 
surface  which  is  directly  exposed  to  the  sun  has 
been  thought  to  be  highly  heated,  possibly  to  the 
degree  of  boiling  water,*  but  this  is  now  considered 
very  improbable. 

Whether  or  not  the  moon  radiates  any  heat  to  the 
earth  has  long  been  a  mooted  question.  The  best 
authorities,  at  present,  estimate  the  average  heat  of 
the  moonbeam  at  about  -sso}^-^  that  we  receive  from 
the  sun,  or  sufficient  to  raise  the  temperature  of  a 
sensitive  black-bulb  thermometer  j,-^q  of  a  degree. 


*  Prof.  Langley  is  now  engaged  in  an  exhaustive  series  of  experiments  upon  this  sub- 
ject, using  his  famous  "bolometer" — an  instrument  capable  of  detecting  a  difference  of 
.00001°  C.  The  result  of  his  observations  upon  Mount  Whitney  (18«1)  showed  that  "  mer- 
cury would  remain  a  solid  under  the  vertical  rays  of  a  tropical  sun  were  radiation  into 
space  wholly  unchecked,  and  that  the  temperature  of  a  planet  may,  and  not  improlably 
does,  depend  far  less  upon  its  neighborhood  tf),  or  remoteness  from,  the  sun,  than  upon 
the  constitution  of  its  atmosphere."  As  the  moon  has  no  air-blanket,  it  is  therefore  very 
doubtful  whether  its  surface  ever  reaches  a  temperature  of  —100°  F. 


1^ 


tflE  SOLAR   SYSTEM. 


It  would  be  absurd  to  suppose  that  this  slight  amount 
of  heat  can  ha-ve  any  appreciable  effect  upon  tlie 
weather. 

Center  of  Gravity. — It  is  thought  that  possibly  the 
center  of  gravity  of  the  moon  is  not  exactly  at  her 
center  of  magnitude,  but  about  thirty-three  miles 
beyond,  the  lighter  half  being  toward  us.  If  that  be 
so,  this  side  is  equivalent  to  a  mountain  of  that 
enormous  height ;  and  if  water  and  air  exist  upon 
the  moon,  they  cannot  remain  on  this  hemisphere, 
but  must  be  confined  to  the  side  which  is  forever 
hidden  from  our  view. 

Atmosphere  of  the  Moon. — The  existence  of  an 
atmosphere  upon  our  satellite  is  at  present  an  open 
question.  If  there  be  any,  it  must  be  extremely 
rarefied,  perhaps  as  much  so  as  that  in  the  vacuum 
obtained  in  the  receiver  of  our  best  air-pumps. 
Appearance  of  the  Earth  to  Lunarians. — If  there 

be  any  lunar  inhabitants 
on  the  side  toward  us,  the 
earth  must  present  to 
them  all  the  phases  which 
their  world  exhibits  to  us, 
only  in  a  reverse  order. 
When  we  have  a  new 
moon,  they  have  a  full 
earth,  a  bright  full-orbed 
moon  fourteen  times  as 
large  as  ours.  The  lunar 
inhabitants  upon  the  side 
opposite  to  us  of  course 
never  see  our  earth,  unless  they  take  a  journey  to 


F'a.  f'. 


Appeararux  of  the  Earth  as  seen  frorn  the 
Moon. 


THE   MOON.  12'? 

the  regions  from  whence  it  is  visible,  to  behold  this 
wonderful  spectacle.  Those  living  near  the  limbs  of 
the  disk  might,  however,  on  account  of  the  lihra- 
tions,  get  occasional  glimpses  of  it  near  their  hori- 
zon. 

The  Earth-Shine. — For  a  few  days  before  and  after 
new  moon,  we  may  distinguish  the  outline  of  the 
unillumined  portion  of  the  moon.  In  England,  it  is 
popularly  known  as  "the  old  moon  in  the  new 
moon's  arms."  This  reflection  of  the  earth's  rays 
must  serve  to  keep  the  lunar  nights  quite  light,  even 
in  new  earth. 

Phases  of  the  Moon. — The  phases  of  the  moon 
show  conclusively  that  it  is  a  dark  body,  which 
shines  by  reflecting  the  light  it  receives  from  the 
sun.  Let  us  compare  its  various  appearances  with 
the  positions  indicated  in  the  figure. 

(1.)  We  see  the  moon  as  a  delicate  crescent  in  the 
west  just  after  sunset,  as  she  emerges  from  the  sun's 
rays  at  conjunction.  She  soon  sets  below  the  hori- 
zon. Half  of  the  surface  is  illumined,  but  only  a 
slender  edge  with  the  horns  turned  from  the  sun  is 
visible  to  us.  Each  night  the  crescent  broadens,  the 
moon  recedes  about  l'S°  further  from  the  sun,  and 
sets  correspondingly  later,  until  at  quadrature  half 
of  the  enlightened  hemisphere  is  turned  toward  us, 
and  the  moon  is  said  to  be  in  her  first  quarter. 

(2.)  The  moon,  continuing  her  eastern  progress 
round  the  earth,  becomes  gibbous*  in  form,  and, 
about  the  fifteenth  day  from  new  moon,  reaches  the 
point  in  the  heavens  directly  opposite  to  that  which 

*  Gibbous  means  more  than  a  half  and  less  tliau  the  whole  of  a  circle. 


i2J 


THE  SOLAR  SYSTEM. 


the  sun  occupies.  She  is  then  in  opposition,  the 
whole  of  the  illumined  side  is  turned  toward  us,  and 
we  have  a  full  moon.   She  is  on  the  meridian  at  mid- 

Fig.  BO. 


Phases  of  the  Moon. 

night,  and  so  rises  in  the  east  as  the  sun  sets  in  the 
west,  and  vice  versa. 


THE   MOON.  129 

(3.)  The  moon,  passing  on  in  her  orbit  from  oppo- 
sition, presents  phases  reversed  from  those  of  the 
second  quarter.  The  proportion  of  the  illumined 
side  visible  to  us  gradually  decreases  ;  she  becomes 
gibbous  again ;  rises  nearly  an  hour  later  each  even- 
ing, and  in  the  morning  lingers  high  in  the  western 
sky  after  sunrise.  She  now  comes  into  quadrature, 
and  is  in  her  third  quarter. 

(4).  From  the  third  quarter,  the  moon  turns  her 
enlightened  side  from  us  and  decreases  to  the 
crescent  form  again ;  as,  however,  the  bright  hemi- 
sphere constantly  faces  the  sun,  the  horns  are 
pointed  toward  the  west.  She  is  now  seen  as  a 
bright  crescent  in  the  eastern  sky  just  before  sun- 
rise. At  last,  the  illumined  side  is  completely  turned 
from  us,  and  the  moon  herself,  coming  into  conjunc- 
tion with  the  sun,  is  lost  in  his  rays.  To  accomplish 
this  journey  through  her  orbit  from  new  moon 
to  new  moon  again,  has  required  29|^  days — a  lunar 
month. 

Moon  Runs  High  or  Low. — All  have,  doubtless, 
noticed  that,  in  the  long  nights  of  winter,  the  full 
moon  is  high  in  the  heavens,  and  continues  a  long 
time  above  the  horizon  ;  while  in  midsummer  she  is 
low,  and  remains  a  much  shorter  time  above  the 
horizon.  This  is  a  wise  plan  of  the  Creator,  which  is 
seen  yet  more  clearly  in  the  arctic  regions.  There, 
the  moon,  during  the  long  summer  day  of  six 
months,  is  above  the  horizon  only  her  first  and  fourth 
quarters,  when  her  light  is  least ;  but  during  the 
tedious  winter  night  of  equal  length,  she  is  continu- 
ally above  the  horizon   for  lier  second  and    third 


130  THE  SOLAR  SYSTEM. 

quarters.  Thus,  in  polar  regions,  the  moon  is  never 
full  by  day,  but  is  always  full  every  month  in  the 
night. 

We  can  easily  understand  these  phenomena  when 
we  remember  that  the  new  moon  is  in  the  same 
quarter  with,  and  the  full  moon  is  in  the  opposite 
quarter  from,  the  sun.  AVhen,  therefore,  the  sun 
sinks  low  in  the  southern  sky  the  full  moon  rises 
high,  and  when  the  sun  rises  high  the  full  moon 
sinks  low. 

Harvest  Moon. — While  the  moon  rises,  on  the 
average,  50  m.  later  each  night,  the  exact  time 
varies  from  less  than  half  an  hour  to  a  full  hour. 
Near  the  time  of  the  autumnal  equinox  the  moon,  at 
her  full,  rises  about  sunset  for  a  number  of  nights 
in  succession.  This  produces  a  series  of  brilliant 
moonlight  evenings.  It  is  the  time  of  harvest  in 
England,  and  hence  has  there  received  the  name  of 
the  Harvest  Moon.  In  the  following  month  (October), 
the  same  occurence  takes  place ;  it  is  then  termed  the 
Hunter's  Moon. 

The  cause  of  this  phenomenon  lies  in  the  fact  that 
the  moon's  path  is  variously  inclined  to  the  horizon 
at  different  seasons  of  the  year.*  When,  at  the  time 
of  rising,  the  full  moon  is  near  the  vernal  equinox, 
the  angle  her  path  makes  with  the  horizon  is  least, 
and  when  she  is  near  the  autumnal  equinox  it  is 
greatest.  In  the  former  case,  the  moon,  moving 
eastward  each  day  about  13°,  will  descend  but  little 
below  the  horizon,   and  so  for  several  successive 

*  Besides  this  reason,  we  should  remember  that  the  motion  of  the  moon  is  slowest 
at  apogee  and  fastest  at  perigee.     (See  note,  p.  302.) 


THE  MOON. 


131 


Fig.  51. 


evenings  will  rise  at  about  the  same  hour.     In  the 

latter,  she  will  descend  much  further  each  day  and 
thus  will  rise  much  later  each  night.  The  least  pos- 
sible variation  in  the  hour  of  rising  is  17  minutes, — 
the  greatest  is  1  hour  and  16  minutes. 

In  Figure  51,  let  S  represent  the  sun  ;  E,  the  earth  ; 
M,  the  moon  ;  C  F,  the  moon's  path  around  the  earth 
when  the  autumnal  equinox  is  in  the  eastern  horizon  ; 
E  D,  when  the  vernal  equinox  is  in  the  eastern  hori- 
zon ;  A  M  B  S,  the  horizon  ;  and  M  cZ  =M  &  =  13°,  the 
distance  the  moon 
moves  each  day. 
When  passing 
along  the  path  C 
F,  the  moon  sinks 
below  the  horizon 
the  distance  a  h, 
and  when  moving 
along  the  path  E 
D,  only  the  dis- 
tance c  d.  It  is 
obvious  that  be- 
fore the  moon  can 
rise  in  the  former 
case,  the  horizon 

must  be  depressed  the  distance  a  b,  and  in  the  latter 
only  c  d  ;  and  the  moon  will  rise  each  evening  cor- 
respondingly later  in  the  one  and  earlier  in  the  other. 

Cause  of  "Dry  Moon,"  and  "Wet  Moon." — At  new 
moon,  when  the  bright  crescent  lies  nearly  perpen- 
dicular to  the  horizon,  the  moon  is  popularly  called 
a  wet  moon,  and  when  it  is  almost  horizontal,  the 


The  Harvest  Moon. 


132  THE  SOLAR  SYSTEM. 

moon  is  termed  a  dry  moon.  The  cause  of  this 
change  in  the  crescent  is  astronomical,  and  not 
meteorological.  The  form  of  the  crescent  has  there- 
fore no  connection  with  the  weather.  A  little  reflec- 
tion will  show  us  that  the  horns,  or  cusps,  of  the 
new  moon  must  point  from  the  sun.  As  the  ecliptic 
(from  which  the  moon's  path  varies  but  slightly)  is 
differently  inclined  to  the  horizon  at  various  times 
of  the  year,  this  will  give  the  crescent  a  different 
position  with  reference  to  the  horizon  (p.  29). 

Nodes. — The  orbit  of  the  moon  is  inclined  to  the 
ecliptic  about  5°,  the  points  where  her  path  crosses 
it  being  termed  nodes.  The  ascending  node  ( 8 )  is 
the  place  where  the  moon  crosses  in  coming  above 
the  ecliptic,  or  toward  the  north  star  ;  the  descending- 
node  ( 3 )  is  where  it  passes  below  the  ecliptic.  The 
imaginary  line  connecting  these  two  points  is  called 
the  ''line  of  the  nodes." 

Occultation. — The  moon,  in  the  course  of  her 
monthly  journey  round  the  earth,  frequently  passes 
in  front  of  the  stars  or  planets,  which  disappear  on 
one  side  of  her  disk  and  reappear  on  the  other.  This 
is  termed  an  occultation,  and  is  of  practical  use  in 
determining  the  difference  of  longitude  between 
various  places  on  the  earth. 

Lunar  Seasons;  Day  and  Night,  Etc. — As  the 
moon's  axis  is  so  nearly  perpendicular  to  her  orbit, 
she  cannot  have  any  change  of  seasons.  During 
nearly  fifteen  of  our  days,  the  sun  pours  down  his 
rays  unmitigated  by  any  atmosphere  to  temper  them. 
To  this  long,  torrid  day  succeeds  a  night  of  equal 
length  and  polar  cold. 


Fig.  es 


134  THE  SOLAK  SYSTEM. 

How  strange  the  lunar  appearance  would  be  to  us ! 
The  disk  of  the  sun  seems  sharp  and  distinct.  The 
sky  is  black  and  overspread  with  stars  even  at  mid- 
day. There  is  no  twilight,  for  the  sun  bursts 
instantly  into  day,  and,  after  a  fortnight's  glare,  as 
suddenly  gives  place  to  night ;  no  air  to  conduct 
sound  ;  no  clouds  ;  no  winds  ;  no  rainbow ;  no  blue 
sky  ;  no  gorgeous  tinting  of  the  heavens  at  sunrise 
and  sunset ;  no  delicate  shading  ;  no  soft  blending 
of  colors,  but  only  sharp  outlines  of  sun  and  shade.  * 

The  nights  of  the  visible  hemisphere  must  be 
brilliantly  illuminated  by  the  earth,  whose  phases 
"serve  well  as  a  clock — a  dial  all  but  fixed  in  the 
same  part  of  the  heavens,  like  an  immense  lamp, 
behind  which  the  stars  slowly  defile  along  the  black 
sky." 

Telescopic  Features. — Even  with  the  naked  eye, 
we  see  on  the  moon's  surface  bright  spots  (the  sum- 
mits of  lofty  mountains,  gilded  by  the  first  rays  of 
the  sun),  and  darker  portions — Ioav  plains  yet  lying 
in  comparative  shadow.  The  telescope  reveals  to 
us  a  region  torn  and  shattered  by  fearful  though 
now    ext"^ctt    volcanic    action.      Everywhere   the 


*  The  moon  is  a  fossil  world,  an  ancient  cinder,  a  ruined  habitation  perpetuated  only 
to  admonish  the  earth  of  her  own  impending  fate,  and  to  teach  her  occupants  that  another 
home  must  be  provided,  whicli  frost  and  decay  can  never  invade.  The  moon  was  once 
the  seat  of  all  the  varied  and  intense  activities  that  now  characterize  the  surface  of  our 
earth.  At  one  time  its  physical  condition  was  like  that  of  tlie  parent  eartli  from  which 
it  had  just  been  seiiarated  :  but,  being  smaller,  it  cooled  faster,  and  its  geologic  periods 
were  coiTcsjiondingly  shorter.  Its  life-age  was  perhaps  reached  while  the  earth  was  yet 
glowing.— Read  Wini-hell's  Geology  of  tlie  Stars. 

t  Several  distinguished  astronomers  assert,  however,  that  the  crater  Linnx-us  lias 
undergone  noticeable  transformations.  Its  sides  seem  to  liave  fallen  in,  and  the  interior 
to  have  become  filled  up,  as  if  by  a  new  eruption.  It  is  said  to  present  an  appearance 
similar  to  that  of  the  Sea  of  Serenity.  Other  marked  changes  are  said  to  have  been 
discovered  from  time  to  time,  on  the  moon's  surface^  but  they  are  not  generally  ac^ 


THE  MOON. 


135 


Fig.  53. 


Telescopic  View  of  the  Mootj, 


136 


THE  SOLAR  SYSTEM. 


crust  is  pierced  by  craters,  whose  irregular  edges 
and  rents  testify  to  the  convulsions  our  satellite  has 
undergone. 

Mountains. — The  heights  of  more  than  1,000  of 
the  lunar  mountains  have  been  measured,  some  of 
which  exceed  25,000  feet.  When  the  sun's  rays  strike 
one  of  these  mountains  obliquely,  the  shadow  is  as 
distinctly  perceived  as  that  of  an  upright  staff  when 
placed  opposite  the  sun.  Some  of  the  elevations  are 
insulated  peaks  that  shoot  up  from  the  center  of 
circular  plains  ;  others  are  mountain  ranges  extend- 


CopenUtug. 


ing  hundreds  of  miles.  Most  of  the  lunar  heights 
have  received  names  of  men  distinguished  in  science. 
Thus  we  find  Plato,  Aristarchus,  Copernicus,*  Kepler, 

credited.  For  an  interesting  discussian  of  this  subject,  read  a  chapter  entitled  "A  Xew 
Crater  in  the  Moon,"  in  Proctor's  Poetry  of  Astronomy. 

*  This  is  one  of  the  grandest  of  the  lunar  craters.  It  is  situated  on  the  tip  of  the  nose 
of  the  "  Man  in  the  Moon."  Its  diameter  is  forty-six  miles,  and  its  encircling  rampart 
rises  12,000  feet  above  the  interior  plateau,  in  tjie  midst  of  which  $t^nds  a  group  ci 
fpnps,  one  2,400  feet  in  heighj. 


$HE  MOON.  131? 

and  Kewton,  associated,  however,  with  the  Apen- 
nines, Carpathians,  etc. 

Gray  Plains,  or  Seas. — These  are  analogous  to 
our  prairies.  They  were  formerly  supposed  to  be 
sheets  of  water,  but  they  exhibit  the  uneven  ap- 
pearance of  a  plain,  instead  of  the  regular  curve  of  a 
sea.  The  former  names  have  been  retained,  and  we 
find  on  lunar  maps  the  Sea  of  Tranquillity,  the  Sea 
of  Nectar,  Sea  of  Serenity,  etc. 

Rills,  Luminous  Bands.  —  The  latter  are  long, 
bright  streaks,  irregular  in  outline  and  extent,  which 
radiate  in  every  direction  from  Tycho,  Kepler,  and 
other  mountains  ;  the  former  are  similar,  but  are 
sunken,  and  have  sloping  sides,  and  were  at  first 
thought  to  be  ancient  river-beds.  Their  nature  is  a 
mystery. 

Craters  constitute  the  most  curious  feature  of  the 
lunar  landscape.  They  are  of  volcanic  origin,  and 
usually  consist  of  a  cup-like  basin,  with  a  conical 
elevation  in  the  center.  Some  of  the  craters  have  a 
diameter  of  over  100  miles,  and  are  great  walled  plains, 
sunk  so  far  behind  huge,  volcanic  ramparts  that  the 
lofty  wall  surrounding  an  observer  at  the  center 
would  be  beyond  his  horizon.  Other  craters  are 
deep  and  narrow, — as  Newton,  which  is  said  to  be 
about  four  miles  in  depth, — so  that  neither  earth  nor 
sun  is  ever  visible  from  a  great  part  of  the  bottom. 
The  appearance  of  these  craters  is  strikingly  shown 
in  the  accompanying  view  (Fig.  53)  of  the  region  to 
the  southeast  of  Tycho. 


138 


THE  SOLAR  SYSTEM. 


ECLIPSES. 

Eclipse  of  the  Sun.  —  If  the  moon  should  pass 
through  either  node  at  or  near  the  time  of  conjunc- 
tion, or  new  moon,  she  would  necessarily  come  be- 
tween the  earth  and  the  sun,  for  the  three  bodies 
are  then  in  the  same  straight  line.    This  would  cause 

Fig.  ''!">. 


Theory  of  a  Total  and  a  Partial  Eclipse  of  the  Sun. 

an  eclipse  of  the  sun.  If  the  moon's  orbit  were  in 
the  same  plane  as  the  ecliptic,  an  eclipse  of  the  sun 
would  occur  at  every  new  moon  ;  but  as  the  orbit  is 
inclined,  it  can  occur  only  at  or  near  a  node. 

Fid.  5^. 


iln-oii/  uj  an.  Aiiaalar  Kdipic  uf  l/ie  6u:t. 


The  eclipse  may  be  partial,  total,  or  annular. 
-In  Fig.  55,  we  see  where  the  dark  shadow  {umbra) 


ECLIPSES.  139 

of  the  moon  falls  on  the  earth  and  obscures  the 
entire  body  of  the  sun.  To  the  persons  within  that 
region,  there  is  a  total  eclipse;  the  breadth  of  this 
space  is  not  large,  averaging  only  140  miles.  Be- 
yond this  umbra,  there  is  a  lighter  shadow,  penum- 
bra, {pene,  almost;  umbra,  a  shadow),  where  only  a 
portion  of  the  sun's  disk  is  obscured.  Within  this 
region,  there  is  a  partial  eclipse.  To  those  persons 
living  north  of  the  equator  and  of  the  umbra,  the 
eclipse  passes  over  the  lower  limb  of  the  sun  ;  to 
those  south  of  the  umbra,  it  passes  over  the  upper 
limb.'"'  When  the  eclipse  occurs  exactly  at  the  node, 
it  is  said  to  be  centixil.  If  the  eclipse  takes  place 
when  the  moon  is  at  apogee,  her  apparent  diameter 
is  less  than  that  of  the  sun  ;  as  a  consequence,  her 
disk  does  not  cover  the  disk  of  the  sun,  and  the  vis- 
ible portions  of  that  luminary  appear  in  the  form  of 
a  ring  (annulus) ;  hence  there  is  an  annular  eclipse 
in  all  those  places  comprised  within  the  limits  of  the 
cone  of  shadow  prolonged  to  the  earth. 

General  facts  concerning  a  solar  eclipse. — The 
following  data  may  guide  in  understanding  the 
phenomena  of  solar  eclipses. 

(1.)  The  moon  must  be  new. 

(2.)  She  must  be  at  or  near  a  node. 

(3.)  When  her  distance  from  the  earth  is  less  than 
the  length  of  her  shadow,  the  eclipse  will  be  total  or 
partial. 

(4.)  When  lier  distance  is  greater  than  the  length 
of  her  shadow,  the  eclipse  will  be  annular  or  partial. 

*  South  of  tljc  yquatiir  the  revi'i'.se  of  these  ijlieiioineiia  wmilcl  Ijaiipeii. 


140  THE  SOLAR  SYSTEM. 

(5.)  There  can  be  no  eclipse  at  those  places  where 
the  sun  himself  is  invisible. 

(6.)  An  eclipse  is  not  visible  over  the  whole  illu- 
mined side  of  the  earth.  As  the  moon's  diameter  is 
less  than  that  of  the  earth,  her  cone  of  shadow  is  too 
small  to  enshroud  the  entire  globe,  so  that  the  region 
in  which  it  is  total  cannot  exceed  180  miles  in 
breadth.  As,  however,  the  earth  is  constantly  rotat- 
ing on  its  axis  during  the  duration  of  the  eclipse, 
the  shadow  may  travel  over  a  large  surface. 

(7.)  If  the  moon's  shadow  fall  upon  the  earth  when 
she  is  nearing  her  ascending  node,  it  will  sweep 

Fig.  57. 


^^^^^^--      ^^J%J 


Solar  Ecliptic  Limit  (17°). 

across  the  south  polar  regions  :  if,  when  nearing  her 
descending  node,  it  will  graze  the  earth  near  the 
north  pole.  The  nearer  a  node  a  conjunction  occurs, 
the  nearer  the  equatorial  regions  the  shadow  will 
strike. 

(8.)  At  the  equator,  the  longest  possible  duration 
of  a  total  solar  eclipse  is  about  eight  minutes  ;  of  an 
annular,  twelve  minutes.  One  reason  of  the  greater 
length  of  the  latter  is,  that  then  the  moon  is  in 
apogee,  when  she  always  moves  slower  than  in 
perigee.  The  duration  of  total  obscuration  is  great- 
est when  the  moon  is  in  perigee  and  the  sun  in 
apogee ;  for  then  the  apparent  size  of  the  moon  is 
greatest,  and  that  of  the  sun  is  least. 


ECLIPSES. 


141 


Fir;.   .'S. 


(9.)  There  cannot  be  more  than  five  nor  less  than 
two  solar  eclipses  per  year.  A  total  or  an  annular 
eclipse,  in  its  recurrence  at  any  place^  is  exceedingly 
rare.  There  has  been  (according  to  Halley)  only  one 
total  eclipse  visible  at  London  since  1140. 

(10.)  A  solar  eclipse  conies  on  the  western  limb,  or 
edge  of  the  sun,  and  passes  off  on  the  eastern. 

(11.)  The  disk  of  the  sun  is  divided  into  twelve 
digits,  and  the  amount  of  the  eclipse  is  estimated  by 
the  number  of  digits  which  it  covers.  Thus  an 
eclipse  of  six  digits  is  one  in  which  half  the  diam- 
eter of  the  disk  is  concealed. 

Curious  phenomena  attend  a  total  eclipse.  Around 
the  sun  is  seen  a 
beautiful  corona, 
or  halo  of  light, 
like  that  wliich 
painters  give  to  the 
head  of  the  Virgin 
Mary.  Flames  of 
a  rose-red  color 
play  around  the 
disk  of  the  moon. 
When  only  a  mere 
crescent  of  the  sun 
is  visible,  it  seems 
to  resolve  itself 
into  bright  spots 
interspersed    with 

dark  spaces,  having  the  appearance  of  a  string  of 
glittering  beads  (Baily's  Beads). 

The  attendant  circumstances  of   a  total   eclipse 


Eclipse  of  1858. 


U2 


THE   SOLAR   SYSTEM". 


Fid.  50. 


are  of  a  peculiarly  impressive  character.  The  dark- 
ness is  so  dense  that  the  brighter  stars  and  planets 
are  seen,  birds  cease  their  songs  and  fly  to  their 
nests,  flowers  close,  and  the  face  of  nature  assumes 
an  unearthly,  cadaverous  hue,  while  a  sudden  fall 

of  the  temperature 
causes  the  air  to 
feel  damp,  and  the 
grass  to  be  wet  as 
if  from  excessive 
dew.  Orange,  yel- 
low, and  copper 
tints  give  objects 
a  strange  appear- 
ance. ''Men  look 
at  eacl>  other,  and 
behold,  as  it  were, 
corpses. " 

The  ancients  re- 
garded     a     total 
eclipse    with    feel- 
ings of  indescribable  terror,  as  an  indication  of  the 
anger  of  an  offended  Deity,  or  the  presage  of  some 
impending  calamity.*    Even  now,  when  the  causes 


Annular  Eclipse  of  1SS5,  skormaig  Baily's  Beads. 


*  William  of  Malinesbury  thus  connects  tlie  eclipse  of  August  '2,  1133,  with  Henrj-  I., 
who  left  England  on  that  day,  never  to  return  alive  :  "  Tlie  elements  manifested  their 
sorrows  at  this  great  man's  last  departure.  For  the  sun  on  that  day,  at  the  Oth  hour, 
shrouded  his  glorious  face,  as  the  poets  say,  in  hideous  darkness,  agitating  the  hearts 
of  men  by  an  eclipse  :  and  on  the  6th  day  of  the  week,  early  in  the  morning,  there  was 
so  great  an  earthriuake  that  the  ground  appeared  suddenly  to  sink  down  ;  an  horrid 
noise  beiiiR  first  heard  beneath  the  surface." 

The  same  writer,  sjieaking  of  the  total  eclipse  of  March  20,  1140,  says  :  "  During  this 
year,  in  Lent,  on  the  13th  of  the  kalends  of  April,  at  the  Oth  honr  of  the  4th  day  of  the 
week,  there  was  an  eclipse,  tliroughout  England,  as  I  have  heard.  With  us,  indeed,  and 
with  all  our  neighbours,  the  obscuration  of  the  Sun  also  was  so  remarkable,  that  i)ersons 
sitting  at  table,  as  it  then  happened  almost  every  where,  for  it  was  Lent,  at  first  feared 


148 


Corona  seen  in  1S71. 


are  fully  understood,  and  the  time  of  the  eclipse  can 
be  predicted  within   the  fraction  of  a  second,  the 


that  Chaos  was  come  again  :  afterwards  learning  the  cause,  they  went  out  and  beheld 
the  stars  around  the  Sun.  It  was  thought  and  said  by  many,  not  untruly,  that  the  king 
(Steplien)  would  not  continue  a  year  in  the  government." 

Columbus  made  use  of  an  approacliing  eclipse  of  tlic  moon,  which  took  place  March  1, 
1504,  to  relieve  his  fleet,  tljen  in  great  distress  from  want  of  supplies.  As  a  punishment 
to  the  islanders  of  Jamaica,  who  refused  to  assist  him,  he  threatened  to  deprive  them 
of  the  light  of  the  moon.  At  first  they  were  indifferent  to  his  threats,  but  "  when  the 
eclipse  actually  commenced,  tlie  barbarians  vied  with  each  othur  in  the  production  of 
the  necessary  supplies  for  the  Spanish  fleet." 

Among  the  Hindoos  a  singular  custom  is  said  to  exist.  When,  during  a  solar  eclipse, 
the  black  disk  of  our  satellite  begins  slowly  to  advance  over  the  sun,  the  natives  believe 
that  some  terrific  monster  is  gradually  devouring  it.  Thereupon  they  lieat  gongs,  and 
rend  the  air  with  screams  of  terror  and  shouts  of  vengeance.     For  a  time  their  frantic 


144  THE  SOLAR  SYSTEM. 

change  from  broad  daylight  to  almost  instantaneous 
gloom  is  overwhelming,  and  inspires  with  awe  even 
the  most  careless  observer.     (See  note,  p.  303.) 

The  Saros. — The  nodes  of  the  moon's  orbit  are  con- 
stantly moving  backward.  They  complete  a  revolu- 
tion around  the  ecliptic  in  about  18^  years.  Now 
the  moon  makes  2-23  synodic  revolutions  in  18  years 
and  10  days  ;  the  sun  makes  19  revolutions  with  re- 
gard to  the  lunar  nodes  in  about  the  same  time. 
Hence,  in  that  period,  the  sun,  the  moon,  and  the 
nodes  will  be  in  nearly  the  same  relative  position. 
If,  then,  we  :qeckon  18  years  and  10  days  from  any 
eclipse,  we  shall  find  the  time  of  its  repetition. 

This  method  was  discovered,  it  is  said,  by  the  Chal- 
deans. The  ancients  were  enabled,  by  this  means, 
to  predict  eclipses,  but  it  is  considered  too  inaccurate 
by  modern  astronomers. 

Metonic  Cycle. — The  Metonic  Cycle  (sometimes 
confounded  with  the  Saros)  was  not  used  for  fore- 
telling eclipses,  but  for  ascertaining  the  age  of  the 
moon  at  a  given  period.  It  consists  of  nineteen 
tropical  years,*  during  which  time  there  are  235  new 
moons  ;  so  that,  at  the  end  of  this  period,  the  new 
moons  will  recur  at  seasons  of  the  year  correspond- 
ing to  those  of  the  preceding  cycle.  By  registering, 
therefore,  the  exact  days  of  any  cycle  at  which  the 

efforts  seem  futile  and  the  eclipse  still  progresses.  At  length,  however,  the  increasing 
uproar  reaches  the  voracious  monster  ;  he  appears  to  pause,  and  then,  like  a  fish  reject- 
ing a  nearly  swallowed  bait,  gradually  disgorges  the  fiery  mouthful.  Wlien  the  sun  is 
quite  clear  of  the  great  dragon's  mouth,  a  shout  of  joy  is  raised,  and  the  poor  natives 
disperse,  delighted  to  think  that  they  have  so  successfully  relieved  their  deity  from  his 
impending  peril. 

*  A  tropical  year  is  the  interval  between  two  successive  returns  of  the  sun  to  the 
vernal  equinox. 


PLATE   in. 


Various  Forms  qf  Solar  Prominmces.    (See  pp.  53,  141,  262.) 


ECLIPSES.  145 

new  and  full  moons  occur,  such  a  calendar  shows  on 
what  days  these  events  will  happen  in  succeeding 
cycles. 

Since  the  appointment  of  games,  feasts,  and  fasts 
has  been  made  very  extensively,  both  in  ancient  and 
modern  times,  according  to  new  or  full  moons,  such 
a  calendar  becomes  very  convenient  for  finding  the 
day  on  which  the  required  new  or  full  moon  takes 
place.  Thus,  if  a  festival  were  decreed  to  be  held  in 
any  given  year  on  the  day  of  the  first  full  moon  after 
the  vernal  equinox  :  find  what  year  it  is  of  the  lunar 
cycle,  then  refer  to  the  corresponding  year  of  the 
preceding  cycle,  and  the  day  will  be  the  same.  The 
Golden  Number,  a  term  still  used  in  our  almanacs, 
denotes  the  year  of  the  lunar  cycle.  Four  is  the 
golden  number  for  1884. 

Fig.  6S. 


Ecli2Jse  oftlie  Moon. 


An  Eclipse  of  the  Moon  is  caused  by  the  passing 
of  the  moon  into  the  shadow  of  the  earth,  and  hence 
can  take  place  only  at  full  moon — opposition.  As 
the  moon's  orbit  is  inclined  to  the  ecliptic,  her  path 
is  partly  above  and  partly  below  the  earth's  shadow  ; 
thus  an  eclipse  of  the  moon  can  take  place  only  at 
or  near  one  of  the  nodes.  In  Fig.  63,  the  umbra  is 
represented  by  the  space  between  the  lines  IS,  c  and 
7 


146  THE   SOLAR  SYSTEM. 

I  b  ;  outside  of  this  is  the  penumbra,  where  the  earth 
cuts  off  the  light  of  only  a  portion  of  the  sun.  The 
moon  enters  the  penumbra  of  the  earth  at  a, — this  is 
termed  her  first  contact  ivith  the  penumbra  ;  next  she 
encounters  the  dark  shadow  of  the  earth  at  b, — this  is 
called  the  first  contact  ivith  the  umbra  j  she  then 
emerges  from  the  umbra  at  c, — the  second  contact 
tcith  the  umbra;  finally,  she  touches  the  outer  edge 
of  the  penumbra  at  d, — the  second  contact  tcith  the 
penumbra.  Since  the  earth  is  so  much  larger  than 
the  moon,  the  eclipse  can  never  be  annular;  as, 
however,  the  eclipse  may  occur  a  little  above  or  be- 
low the  node,  the  moon  may  only  partly  enter  the 
earth's  shadow,  either  on  its  upper  or  lower  limb. 
From  the  first  to  the  last  contact  with  the  penumbra, 
five  hours  and  a  half  may  elapse. 

Total  eclipses  of  the  moon  are  rarer  events  than 
those  of  the  sun,  since  the  lunar  ecliptic  limit  is  only 
about  12° ;  yet  they  are  more  frequently  seen  by  us, 
(1)  because  each  one  is  visible  over  the  entire  unil- 
lumined  hemisphere  of  the  earth,  and  also  (2)  be- 
cause by  the  diurnal  rotation  during  the  long  dura- 
tion of  the  eclipse,  large  areas  may  be  brought 
within  its  limits.  So  it  will  happen  that  while  the 
inhabitants  of  one  district  witness  the  eclipse  through- 
out its  continuance,  those  of  other  regions  merely 
see  its  beginning,  and  others  only  its  termination. 

The  moon  does  not  completely  disappear  even  in 
total  eclipses.  The  cause  of  this  lies  in  the  refraction 
of  the  solar  rays  in  traversing  the  lower  strata  of 
the  earth's  atmosphere :  they  are  analyzed,  and 
purple  our  moon   with    the  tints   of    sunset,     The 


THE   TIDES.  147 

amount  of  refraction  and  the  color  depend  upon  the 
state  of  the  air  at  the  time. 


THE     TIDES. 


Description. — Twice  a  day,  at  intervals  of  about 
twelve  hours  and  twenty-five  minutes,  the  water  be- 
gins to  set  in  from  the  ocean,  beating  the  pebbles 
and  the  foot  of  the  rocky  shore,  and  dashing  its 
spray  high  in  air.  For  about  six  hours,  it  climbs  far 
up  on  the  beach,  flooding  the  low  lands  and  trans- 
forming creeks  into  rivers.  The  instant  of  high-water 
or  flood-tide  being  reached,  the  water  begins  to 
descend,  and  the  ehh  succeeds  the  flow.  The  water, 
however,  falls  somewhat  slower  than  it  rises. 

Fig.  63. 


Spring  Tide. 

The  Tides  are  Caused  by  a  great  wave,  which, 
raised  by  the  moon's  attraction,  follows  her  in  her 
course  around  the  earth.*    The  sun,  also,  aids  some- 

*  Prof.  Ball,  Royal  Astronomer  of  Ireland,  elainis  that  once  the  moon  was  nearer  the 
earth  than  now  ;  the  day  and  the  month  were  equal,  eacli  three  hours  long.  At  40,000 
miles  distance,  the  moon  was  a  greater  tide  iiroducer  l>y  216  times.  As  the  moon  receded 
from  the  earth,  both  revolved  more  slowly.  At  the  ju'csont  time,  27  eartli  rotations 
equal  one  moon  rotation.  This  lias  remained,  and  will  remain,  sensibly  tnie,  for  thou- 
sands of  years.  But  the  friction  of  tlie  tides  will,  in  the  far  future,  lengthen  the  day  to 
pqual  57  of  our  present  days, — a  condition  tliat  will  then  last  for  ages, 


148  THE  SOLAR  SYSTEM. 

what  in  producing  this  effect ;  but  as  the  moon  is  400 
times  nearer  the  earth,  her  influence  is  far  greater.  * 

As  the  waters. are  free  to  yield  to  the  attraction  of 
the  moon,  she  draws  them  away  from  C  and  D  and 
they  become  heaped  up  at  A.  The  earth,  being 
nearer  the  moon  than  the  waters  on  the  opposite 
side,  is  more  strongly  attracted,  and  so,  being  drawn 
away  from  them,  they  are  left  heaped  up  at  B.  As 
the  result,  high-water  is  produced  at  A  by  the  water 
Doing  pulled  from  the  earth,  and  at  B  by  the  earth 
being  pulled  from  the  water. 

The  influence  of  the  moon  requires  a  little  time  to 
produce  its  full  effect ;  hence  high- water  does  not 
occur  at  any  place  when  the  moon  is  on  the  me- 
ridian, but  a  few  hours  after.  As  the  moon  rises 
about  fifty  minutes  later  each  day,  there  is  a  corre- 
sponding difference  in  the  time  of  high-water. 
While,  however,  the  lunar  tide-wave  thus  lags  about 
fifty  minutes  every  day,  the  solar  tide  occurs  uni- 
formly at  the  same  time.  They  therefore  steadily 
separate  from  each  other.  At  one  time,  they  coin- 
cide, and  high-water  is  the  sum  of  the  lunar  and 
solar  tides ;  at  other  times,  high-water  of  the  solar 
tide  and  low-water  of  the  lunar  tide  occur  simul- 
taneously, and  high-water  is  then  the  difference  be- 
tween the  lunar  and  solar  tides,  f 

*  The  whole  attraction  of  the  moon  is  only  tJo  that  of  the  sun  :  yet  her  influence  in 
producing  the  tides  and  precession  is  greater,  because  tliat  depends  not  upon  the  entire 
attraction  either  exerts,  but  upon  the  difference  between  their  attraction  upon  the 
earth's  center  and  upon  the  earth's  nearest  surface.  For  the  moon,  on  account  of  her 
nearness,  the  proportion  of  the  distance  of  these  parts  is  treble  that  of  the  sun,  and 
hence  her  greater  eflect. 

t  We  should  bear  in  mind  the  philosophical  truth,  that  the  tide  in  the  open  sea  does 
not  consist  of  a  progressive  movement  of  t)ie  water  itself,  but,  only  of  the  form  of  the 
>vave.— P^j/siw,  p-  lOlt 


THE  TIDES.  149 

Causes  that  modify  the  tides. — At  new  and  at 
full  moon  (the  syzygies)  the  sun  acts  with  the  moon 
(Fig.  63)  in  elevating  the  waters ;  this  produces  the 
highest,  or  Spring-tide.  In  quadrature  (Fig.  64),  the 
sun  tends  to  diminish  the  height  of  the  water  :  this 
is  called  Neap-tide.  When  the  moon  is  in  perigee, 
her  attraction  is  stronger ;  hence  the  flood-tide  is 
higher,  and  the  ebb-tide  is  lower  than  at  other  times. 


Neap  Tide. 

This  remark  applies  also  to  the  sun.  The  height  of 
the  tide  also  varies  with  the  declination  of  the  sun 
and  the  moon, — the  highest  or  equinoctial  tides  tak- 
ing place  at  the  equinoxes,  if,  when  the  sun  is  over 
the  equator,  the  moon  also  happens  to  be  very  near 
it :  the  lowest  occur  at  the  solstices.  The  force  and 
the  direction  of  the  winds,  the  shape  of  the  coast, 
and  the  depth  of  the  sea  greatly  complicate  the  ex- 
planati'on  of  local  tides. 

Height  of  the  tide  at  different  places. — In  the 
open  sea,  the  tide  is  hardly  noticeable,  the  water 
sometimes  rising  not  higher  than  a  foot ;  but  where 
the  wave  breaks  on  the  shore,  or  is  forced  up  into 


150  THE  SOLAR  SYSTEM. 

bays  or  narrow  channels,  it  is  very  conspicuous. 
The  difference  between  ebb  and  flood  neap-tide  at 
New  York  is  over  three  feet,  and  that  of  spring-tide 
over  five  feet ;  while  at  Boston  it  is  nearly  double 
this  amount.  A  headland  jutting  out  into  the  ocean 
will  diminish  the  tide ;  as,  for  instance,  off  Cape 
Florida,  where  the  average  height  is  only  one  and  a 
half  feet.  A  deep  bay  opening  up  into  the  land  like 
a  funnel  will  converge  the  wave,  as  at  the  Bay 
of  Fundy,  where  it  rolls  in,  a  great,  roaring  wall 
of  water  sixty  feet  high,  frequently  overtaking  and 
sweeping  off  men  and  animals.*  The  tide  sets  up 
against  the  current  of  rivers,  and  often  entirely 
changes  their  character ;  for  example,  the  Avon 
at  Bristol  is  a  shallow  ditch,  but  at  flood-tide  it 
becomes  a  deep  channel  navigable  by  the  largest 
Indiamen. 


V.     MARS. 

The  god  of  war.    Sign,  S  ,  shield  and  spear. 

Description. — Passing  outward  in  our  survey  of  the 
solar  system,  we  next  meet  with  Mars.  This  is  the 
first  of  the  superior  planets,  and  the  one  most  like 
the  earth.  It  appears  to  the  naked  eye  as  a  bright 
red  star,  rarely  scintillating,  and  shining  with  a 
steady  light,  which  distinguishes  it  from  the  fixed 


The  tide-wave  ascends  the  Hudson  Kiver  at  about  the  same  speed  as  the  steam- 
boats ;  at  Albany  it  reaches  a  height  of  a  little  over  two  feet. 


MARS. 


151 


stars.*  At  conjunction  its  apparent  diameter  is  only 
about  4";  but  once  in  about  two  years  it  conies  into 
opposition  with  the  sun,  when  its  diameter  may  in- 
crease to  30".  At  intervals  of  nearly  15  years,  this 
occurs  when  the  planet  is  in  perihelion  and  the  earth 
in  aphelion.  Mars  then  shines  with  a  brilliancy 
rivalling  that  of  Jupiter  himself,  f 


Fit}.  65. 


DlaiiiAiUr  q/  Mars  at  Exlienic,  Mean,  and  Leant  Distances. 

Motion  in  Space. — Mars  revolves  around  the  Sun 
at  a  mean  distance  of  about  141,000,000  miles.  Its 
orbit  is  sufficiently  flattened  to  bring  it  at  perihelion 
26,000,000  miles  nearer  that  luminary  than  when  in 
aphelion.  Its  motion  varies  in  different  portions  of 
its  orbit,  but  the  average  velocity  is  about  fifteen 
miles  per  second.  The  Martian  day  is  37  min.  longer 
than  ours,  and  the  year  contains  about  608  Martian 
days,  equal  to  087  terrestrial  days  (nearly  two  years). 

Distance   from  Earth. — When  in  opposition,   the 


*  Its  ruddy  appearance  has  led  to  its  being  celebrated  among  all  nations.     The  Jews 
gave  it  the  appellation  of  "  blazing,"  and  it  bore  in  other  languages  a  similar  name. 

\  The  next  favorable  opposition  will  occur  in  1892. 


152  THE  SOLAR  SYSTEM. 

distance  of  Mars  is  (like  that  of  all  the  superior 
planets)  the  difference  between  the  distance  of  the 
planet  and  that  of  the  earth  from  the  Sun :  at  con- 
junction, it  is  the  sum  of  these  distances.  If  the 
orbits  were  circular,  these  distances  would  be  the 
same  at  every  revolution.  The  elliptical  figure,  how- 
ever, occasions  much  variation.  Thus,  if  Mars,  at 
opposition,  be  in  perihelion  while  the  earth  is  in  aphe- 
lion, it  is  removed  from  us  about  34,000,000  miles. 

Dimensions. — The  diameter  of  Mars  is  nearly  4,200 
miles.  *  Its  volume  is  about  1  and  its  density  i  that 
of  the  earth.  A  stone  let  fall  on  its  surface  would 
fall  six  feet  the  first  second.  It  is  somewhat  flat- 
tened at  the  poles,  and  bulged  at  the  equator,  like  our 
globe. 

Seasons. — The  light  and  heat  of  the  sun  at  Mars 
are  less  than  one-half  that  which  we  enjoy.  Its  axis 
is  inclined  about  27',  therefore  its  zones  and  seasons 
do  not  differ  materially  from  our  own  :  its  days, 
also,  as  we  have  seen,  are  of  nearly  the  same  length. 
Since,  however,  its  year  is  equal  to  nearly  two  of 
our  years,  the  seasons  are  lengthened  in  proportion. 

There  must  be  a  considerable  difference  between 
the  temperature  of  its  northern  and  southern  hemi- 
spheres, as  the  former  has  its  summer  when  20,000,000 
miles  further  from  the  sun  than  the  latter :  an  in- 
creased length  of  76  days  may,  however,  be  sufficient 

*  Some  autliors  place  the  diameter  of  Mars  at  about  5,000  miles.  There  is,  also,  a 
discrepancy  as  to  the  other  data  of  this  planet.  Prof.  Hall,  as  the  result  of  his 
observations,  gives  the  density  =  .770  ;  force  of  gravity  =  .37  ;  fall  of  a  body,  1st 
sec.  =  0  feet.  With  the  discovery  of  the  satellites  we  have  now  the  means  of  securing 
exact  results.  The  difficulty  of  observation,  however,  is  shown  from  the  fact  that  "  the 
light  which  falls  upon  the  earth  from  one  of  these  moons  is  about  what  a  man's  hand  on 
which  the  sun  sliines  at  Washington  would  reflect  to  Boston." 


MARS. 


153 


Fig.  66. 


compensation.     It  lias  an  atmosphere  like  our  own, 
loaded  with  clouds. 

Mars  has  two  moons.  *  Our  earth  and  its  moon  pre- 
sent in  the  Martian  evening  sky  a  beautiful  pair  of 
planets,  constantly  remaining  in  close  proximity  to 
each  other,  and  exhibiting 
all  the  phases  which  Mer- 
cury and  Venus  present  to 
us. 

Telescopic  Features. — Un- 
der the  telescope.  Mars  ex- 
hibits slight  phases.  Its 
surface  is  covered  M'^ith  red- 
dish spots,  which  are  be- 
lieved to  be  continents.! 
Other  portions,  of  a  green- 
ish tint,  are  considered  to 
be  bodies  of  water.  The  proportion  of  land  to  water 
on  the  earth  is  reversed  in  Mars.  '^^  Here  every  con- 
tinent is  an  island ;  there  every  sea  is  a  lake  :  but 


w  vj  Mar 


*  Tlie  satellites  of  Mars  were  discovered  in  August,  1S77,  by  Prof.  Hall  of  the  Nava) 
Observatory,  Washington.  The  outer  one  revolves  about  the  planet  in  30  hr.  18  min., 
at  a  distance  of  about  12,300  miles  ;  and  the  inner  one  in  7  hr.  40  min.,  at  a  distance  of 
3,600  miles  (less  than  that  of  remote  cities  on  our  own  continent).  The  inner  moon 
moves  so  much  faster  than  the  rotation  of  Mars  that  to  an  inhabitant  of  that  planet,  the 
moon  would  seem  to  rise  in  the  west  and  set  in  the  east,  passing  through  all  the  phases 
of  our  moon  during  a  single  night.  The  moons  have  been  named  Deimos  and  Phobus, 
or  Dread  and  Terror— the  sons  of  Mars.  The  diameter  of  tliese  little  globes  is  probably 
less  than  15  miles.  For  an  amusing  description  of  such  a  world,  read  "  Living  in  Dread 
and  Terror,"  a  chapter  in  Proctor's  "  Poetry  of  Astronomy." 

t  So  carefully  has  the  surface  of  this  planet  been  studied,  that  a  globe  of  Mars  has 
been  prei)ared  which  is  said  to  be  in  some  respects  more  perfect  than  any  globe  of  the 
earth.  The  different  bodies  of  land  and  water  have  been  named  after  distinguished 
astronomers.  A  characteristic  feature  of  the  seas  is  the  long,  narrow  cliannels.  Schi- 
aparelli,  the  Italian  astronomer,  claims  to  have  discovered  a  number  of  singular  dark 
lines,  now  known  as  "  canals."  They  seem  to  connect  different  bodies  of  water,  and, 
though  without  sufficient  reason,  have  been  by  some  considered  as  the  work  of  the  Mar- 
tian inhabitants. 


154  THE  SOLAR  SYSTEM. 

these,  like  our  own  continents,  are  chiefly  confined 
to  one  hemisphere,  so  that  the  habitable  area  of  the 
two  globes  may  not  differ  so  much  as  the  size  of  the 
planets." 

The  ruddy  color  is  thought  by  Herschel  to  be  due 
to  an  ochery  tinge  in  the  soil ;  by  others  it  is  attrib- 
uted to  peculiarities  of  the  atmosphere  and  clouds. 
Lambert  suggests  that  the  color  of  the  vegetation  on 
Mars  may  be  red  instead  of  green.  There  are  con- 
stant changes  going  on  in  the  brightness  of  the  disk, 
owing,  it  is  supposed,  to  the  variation  of  the  clouds 
of  vapor  in  its  atmosphere.  No  mountains  have  yet 
been  discovered. 

In  the  vicinity  of  the  poles  are  brilliant  white 
spots,  which  are  considered  to  be  masses  of  snow. 
The  " snoiv  zoiies"  apparently  melt  and  recede  with 
the  return  of  summer  in  each  hemisphere,  and  in- 
crease on  the  approach  of  winter.  We  can  thus  from 
the  earth  watch  the  formation  of  polar  ice  and  the 
fall  of  snow, — in  fact,  the  changes  of  the  seasons — on 
the  surface  of  a  neighboring  planet. 


VI.    THE    MINOR    PLANETS. 

Discovery.— Beyond  Mars  there  is  a  wide  interval 
that  was  not  filled  until  the  present  century.  The 
bold,  imaginative  Kepler  conjectured  that  there  was 
a  planet  in  this  space.  This  supposition  was  cor- 
roborated by  Titius's  discovery  of  what  has  since 
been  known  as 

Bode's  Law.— Take  the  numbers  0,  3,  G,  12,  24,  iS, 


THE  Minor  planets,  or  asteroids.  155 

96, 192,  384,  each  of  which,  after  the  second,  is  double 
the  preceding  one.  If  we  add  4  to  each  of  these 
numbers,  we  form  a  new  series  ; 

4,  7,  10,  16,  28,  52,  100,  196,  388. 

A.t  the  time  this  law  was  discovered,  these  numbers 
represented  very  nearly  the  proportionate  distance 
from  the  sun  of  the  planets  then  known,  taking  the 
earth's  distance  as  ten,  except  that  there  was  a  blank 
opposite  28.  This  naturally  led  to  inquiry,  and  a 
systematic  effort  to  solve  the  mystery.  * 

On  the  1st  day  of  January,  1801,  the  nineteenth 
century  was  inaugurated  by  Piazzi's  discovery  of 
the  small  planet  Ceres,  at  almost  the  exact  distance 
necessary  to  fill  the  gap  in  Bode's  series.  The  an- 
nouncement of  other  new  planets  soon  followed, 
until  now  (1885)  there  are  two  hundred  and  forty- 
seven,  with  a  probability  of  more  being  found.  In- 
deed, Leverrier  has  calculated  that  there  may  be 
perhaps  150,000  in  all. 

Description.  —  These  minor  worlds,  or  "pocket 
planets,"  as  Herschel  styled  them,  are  diminutive 
indeed.  The  largest  of  them  is  Vesta,  which  shines 
at  times  as  a  star  of  the  6th  magnitude,  and  can  then 
be  seen  with  the  naked  eye.  f    Those  recently  discov- 

*  It  is  a  curious  fact  that  the  discovery  of  Ceres  sliould  have  been  made  by  an  out- 
sider,  as  Piazzi  did  not  belong  to  the  society  of  24  astronomers  tlien  searching  for  the 
planet.  The  publication  of  Bode's  law  had  little  to  do  with  the  result.  In  fact,  the 
direct  cause  was  an  error  of  the  press  in  putting  an  extra  star  in  Wollaston's  Catalogue, 
and  while  Piazzi  was  looking  for  this  star  he  found  Ceres. 

t  The  small  size  of  the  disks  of  the  minor  planets  defies  exact  measurement.  New- 
comb  makes  Ceres  and  Vesta  the  largest  of  the  group,  with  diameters  between  200  and 
400  miles.  Echo  has  been  assigned  a  diameter  of  17  miles,  or  not  far  from  the  size  of  the 
miniature  moons  of  Mars.  S.n'eral  of  these  little  worlds  have  been  found  but  to  be  lost 
again  ;  while  the  mere  labor  of  tracing  the  movements  of  so  many  tiny  globes  already 
surpasses  the  probable  worth  of  the  results. 


156  THE  SOLAR  SYSTEM. 

ered  are  so  small  that  it  is  difficult  to  decide  which 

is  the  smallest.  A  good  walker  could  easily  m.ake  the 
tour  of  one  in  a  day  ;  a  prairie  farmer  would  need  to 
pre-empt  a  whole  such  world  for  a  cornfield.  *'A 
man  placed  on  one  of  these  tiny  globes  could  leap 
60  feet  high,  and,  in  his  descent,  would  sustain  no 
greater  shock  than  he  does  on  the  earth  from  jump- 
ing or  leaping  a  yard.  *"  These  planets  revolve  around 
the  sun  in  regular  orbits,  comprising  a  zone  about 
100,000,000  miles  in  width.  Their  paths  are  variously 
inclined  to  the  ecliptic;  Massalia's  is  only  41,  while 
that  of  Pallas  rises  34". 

Origin. — A  conjecture  concerning  the  origin  of  these 
bodies  is,  that  they  are  the  fragments  of  a  large 
planet  that,  in  a  remote  antiquity,  was  shivered  to 
pieces  by  some  terrible  catastrophe.  "  One  fact 
seems  above  all  others  to  confirm  the  idea  of  an  inti- 
mate relation  between  these  planets.  It  is  this :  if 
their  orbits  consisted  of  solid  rings,  they  would  be 
found  so  entangled  that  it  would  be  possible,  by 
taking  up  any  one  at  random,  to  lift  all  the  rest." 
The  more  probable  view  is  given  under  the  "  Nebular 
Hypothesis." 

Names  and  Signs.— Ceres,  the  first  discovered,  re- 
ceived the  symbol  9 ,  a  sickle,  as  that  goddess  was 
supposed  to  preside  over  harvests.  Pallas,  the 
second,  named  from  the  goddess  of  wisdom  and  sci- 
entific warfare,  obtained  the  sign  $ ,  the  head  of  a 
spear.  Of  late,  a  simple  circle  with  the  number 
inclosed  has  been  adopted  ;  thus  O  represents  Ceres, 
®  is  the  sign  of  Pallas. 


JUPITER.  15"? 


VII.    JUPITER. 

The    king  of   the  gods.      Sign    7i,   a    liieroglyphic    representation    of  an    eagle, 
"  tlie  bird  of  Jove." 

Description. — From  the  smallest  members  of  the 
solar  system  we  now  pass  to  the  largest  planet — the 
colossal  Jupiter.  Its  peculiar  splendor  and  brilliancy 
distinguish  it  from  the  fixed  stars,  and  vie  even  with 
the  lustre  of  Venus.  It  is  one  of  the  five  planets  dis- 
covered in  primitive  ages.* 

Motion  in  Space. — Jupiter  revolves  about  the  sun 
at  a  mean  distance  of  about  483,000,000  miles.  His 
movement  among  the  fixed  stars  is  slow  and  majestic, 
comporting  well  with  his  vast  dimensions  and  the 
dignity  conferred  by  four  attendant  worlds.  He 
advances  through  the  zodiac  at  the  rate  of  one 
sign  yearly  ;  so  that  if  we  locate  the  planet 
now,  a  year  hence  we  shall  find  it  equally  advanced 
in^he  next  sign.  Yet  slowly  as  he  seems  to  travel 
through  the  heavens,  he  is  bowling  along  through 
space  at  the  enormous  speed  of  nearly  500  miles  per 
minute.  The  Jovian  day  is  equal  to  only  about  ten 
of  our  hours,  while  the  year  is  lengthened  to  about 
12  of  our  years,  comprising  near  10,000  of  his  days. 

Distance  from.  Earth. — Once  in  thirteen  months 
Jupiter  is  in  opposition,  and  his  distance  from  the 
earth  is  measured  by  the  difference  of  the  distances 
of  the  two  bodies  from  the  sun.     At  the  expiration 

'  In  tliose  early  times,  Jupiter  was  supposed  to  be  the  cause  of  storm  and  tempest 
Pliny  thought  that  lightning  owed  its  origin  to  this  planet.  An  old  almanac  of  1368, 
foretelling  the  harmless  condition  of  Jupiter  for  a  certain  month,  says,  "  Jubit  es  hote 
and  moyste  and  does  weel  til  al  thyiiges  and  noyes  nothing." 


m 


TttE  SOLAR  SYSTEM. 


Fig.  €7 


of  half  this  time  he  is  in  conjunction,  and  his  dis- 
tance from  us  is  measured  by  the  sum  of  these 
distances. 

Dimensions.— The  diameter  of  this  planet  is  about 
90,000  miles.  Its  volume  is  1,4:00  times  that  of  the 
earth,  and  much  ex- 
ceeds that  of  all  the 
other  planets  com- 
bined. Seen  at  the 
distance  of  the  moon, 
this  immense  globe 
would  embrace  1,000 
times  the  space  of  the 
full  moon.  Its  den- 
sity is  only  one-quar- 
ter that  of  the  earth  : 
moreover,  its  rapid  ro- 
tation upon  its  axis, 
whereby  a  particle  on 
the  equator  revolves  with  a  velocity  of  473  miles  per 
minute  against  the  earth's  17  miles  per  minute,  must 
produce  a  powerful  centrifugal  force  which  materi- 
ally diminishes  the  weight  of  objects  near  its 
equator.  Consequently,  a  stone  let  fall  on  Jupiter 
would  pass  through  only  about  -42  feet  the  first 
second.  As  a  result  of  this  rapid  rotation,  the 
planet  is  one  of  the  most  flattened  of  any  in  the 
solar  system,  the  equatorial  diameter  exceeding  the 
polar  by  5,000  miles. 

Seasons. — As  the  axis  of  Jupiter  is  but  slightly  in- 
clined from  a  perpendicular  to  the  plane  of .  its  orbit, 
there  is  little  difference  in  the  length  of  his  days  and 


I  ic  of  Jin 


JUPITER.  159 

nights,  which  are  each  of  about  five-hours  duration. 
At  the  poles,  the  sun  is  visible  for  nearly  six  years, 
and  then  remains  set  for  the  same  length  of  time. 
The  seasons  are  but  slightly  varied.  Summer  reigns 
near  the  equator,  while  the  temperate  regions  enjoy 
perpetual  spring.  The  light  and  heat  of  the  sun  are 
only  2r  of  what  we  receive  ;  yet  peculiarities  of  soil 
or  atmosphere  may  compensate  this  difference.  The 
evening  sky  on  Jupiter  must  be  magnificent ;  besides 
the  glittering  stars  which  adorn  our  heavens,  four 
moons,  waxing  and  waning,  each  with  its  diverse 
phase,  illuminate  his  night.  All  the  starry  exhi- 
bition sweeps  through  the  sky  in  five  hours. 

Telescopic  Features. — Jupiter's  Moons. — Through 
the  telescope*  Jupiter  presents  a  beautiful  Copernican 
system  in  miniature.  Four  small  stars — moons — ac- 
company him  in  his  twelve-yearly  revolutions.  From 
hour  to  hour  their  positions  vary,  and  they  seem  to 
oscillate  from  one  side  to  the  other  of  the  planet.  At 
one  time,  there  will  be  two  on  each  side  ;  and  again, 
three  on  one  side,  while  the  remaining  star  is  left 
alone.  They  are  also  frequently  found  to  disappear, 
one,  two,  or  even  three  at  a  time,  and,  more  rarely, 
all  four  at  once. 

These  moons  are  called  by  the  ordinal  numbers, 
reckoning  outward  from  the  planet.  With  an  ordi- 
nary glass,  there  is  nothing  to  distinguish  them 
from  small  stars.     The  Illrd.,  being  the  largest  and 

*  There  are  well-authenticated  instances  on  record  of  their  having  been  seen  by  the 
naked  eye.  Among  others,  the  follo\i'ing  singular  case  is  mentioned.  Wrangle,  the 
celebrated  Russian  traveler,  states  that,  when  in  Siberia,  he  once  met  a  hutiter,  who 
said,  pointing  to  Jupiter,  "  I  have  just  seen  that  star  swallow  a  small  one  and  then 
vomit  it  up  again." 


160 


THE  SOLAR  SYSTEM. 


brightest,  will  generally  be  identified  the  most  easily. 
The  1st.  satellite  appears  to  the  inhabitants  of  the 
planet  almost  as  large  as  our  moon  to  us  ;  the  Ilnd., 
and  Illrd.,  about  half  as  large. 


SATELLITES   OF   JUPITER. 


Mean  distance 
from  Jupiter. 

Diameter. 

Density. 
Water  as  1. 

Sidereal  period. 

I.    lo 

267,380 

425,156 

678,393 

1,192,823 

2,352  m. 
2,000  " 
3,436  " 
2,929  " 

1.12 
2.14 
1.87 
1.47 

I).      11.      .M. 

1    IS    23 
3    13      4 

in.  Ganymede 

IV.  Callisto 

7      3    43 
19    16    32 

It  is  noticeable  that  here  are  four  satellites  revolv- 
ing about  Jupiter,  one  of  them  larger  than  the 
planet  Mercury,  and  each  surpassing  in  size  the 
minor  planets  between  Mars  and  Jupiter.  The 
moons  are  not  only  distinguised  by  their  various 
dimensions,  but  also  by  the  variety  of  their  color. 
The  1st.  and  Ilnd.  have  a  bluish  tint,  the  Illrd.  a 
yellow,  and  the  IVth,  a  reddish  shade.  The  space 
occupied  by  this  miniature  system  is  about  two  and 
a  half  million  miles  in  diameter. 

Eclipse  of  the  Moons. — Jupiter,  like  all  celestial 
bodies  not  self-luminous,  casts  into  space  a  cone  of 
shade.  The  1st.,  Ilnd.,  and  Illrd.  satellites  revolve 
in  orbits  but  very  little  inclined  to  the  plane  of  the 
planet's  orbit.  During  each  revolution,  they  pass 
between  the  Sun  and  Jupiter,  producing  a  solar 
eclipse  ;  and  also,  by  passing  through  the  shadow  of 
the  planet  itself,  cause  to  themselves  an  eclipse 
of  the  sun,  and  to  Jupiter  an  eclipse  of  a  moon. 
The  IVth.  moon  passes  through  a  path  more  in- 
clined, and  therefore  its  eclipses  are  less  frequent ; 


JUPITER. 


161 


instead  of  being  fully  eclipsed,  it  sometimes  just 
grazes  the  shadow.  Through  a  telescope,  we  can  dis- 
tinctly watch  the  disappearance,  or  immersion,  of  the 


Fig.  68. 


Eclipses  and  Occnltations  of  Jupiter's  Moon.'i 


satellites  in  the  planet's  shadow,  their  reappearance, 
or  emersion,  and  also  the  transits  of  their  shadows 
as  round  black  dots  moving  across  the  disk  of  Jupiter. 


162  THE  SOLAR   SYSTEM. 

In  Fig.  68,  we  see  various  positions  of  the  moons  : 
the  1st.  is  eclipsed ;  the  Ilnd.  is  passing  across  the 
disk  of  the  planet  on  which  its  shadow  is  also 
thrown  ;  the  Illrd.  is  just  behind  the  planet,  and  so 
occulted  or  concealed,  while  it  has  not  yet  entered 
the  shadow;  the  IVth.  is  in  view  from  the  earth. 

These  satellites  revolve  with  great  rapidity,  as  is 
necessary  in  order  to  overcome  the  superior  attrac- 
tion of  the  planet  and  prevent  their  being  drawn  to 
its  surface.  The  1st.  goes  through  all  its  phases  in 
If  days:  the  IVth.,  in  less  than  twenty  days.  A 
spectator  on  Jupiter  might  witness,  during  the 
Jovian  year,  4,500  eclipses  of  the  moon  (moons),  and 
about  the  same  nimiber  of  the  sun. 

Velocity  of  Light — By  an  attentive  examination 
of  the  eclipses  of  Jupiter's  moons.  Romer  (a  Danish 
astronomer),  in  1617,  discovered  that  the  motion  of 


light  is  not  instantaneous,  as  was  then  believed.  He 
noticed  that  the  observed  times  of  the  eclipses  were 
sometimes  earlier  and  sometimes  later  than  the 
calculated  times,  according  as  Jupiter  was  nearest 
or  furthest  from  the  earth.  In  Fig.  69,  let  J  represent 
Jupiter  ;  e,  one  of  the  moons  :  S,  the  sun  ;  and  T  and  t, 
different  positions  of  the  earth  in  its  orbit.     When 


JUPITER.  163 

the  earth  is  at  T,  the  eclipse  occurs  16  min.  and  36 
sec.  earlier  than  at  t.  That  interval  of  time  is 
required  for  the  light  to  travel  across  the  earth's 
orbit,  giving  a  velocity  of  about  186,000  miles  per 
second. 

Jupiter's  Belts  are  dusky  streaks  of  varying 
breadth  and  number,  lying  more  or  less  parallel  to 
the  planet's  equator.  A  brighter,  often  rose-colored, 
space  marks  the  equatorial  regions.  The  belts  are 
not  permanent,  but  change  sometimes  in  the  course 
of  a  few  hours.  Occasionally,  only  two  or  three 
broad  belts  are  visible ;  at  other  times,  a  dozen 
narrow  ones  appear.  Often,  spots  are  seen  that  are 
more  lasting  than  the  dark  stripes.*  It  is  now  sup- 
posed that  the  planet  is  enveloped  in  dense  clouds, 
through  which  light  cannot  penetrate,  and  that  the 
globe  itself  is  heated  to  a  high  degree,  and  gives  off 
vapors,  t  The  parallel  appearance  is  doubtless  due 
to  strong  equatorial  currents,  analogous  to  our  trade- 
winds. 

*  In  1878,  a  "Great  Red  Spot"  appeared  in  the  southern  hemisphere  of  Jupiter. 
Its  length  was  estimated  at  8,000  miles,  and  its  breadth  at  2,000  miles.  This  curious 
phenomenon  is  still  visible,  but  much  diminished  in  brightness  (1884). 

t  Jupiter  and  Satimi  are  older  planets  than  the  earth  and  Mars,  but,  being  so  large, 
they  have  cooled  more  slowly,  and  are  yet  only  partially  solidified,  so  that  Jupiter,  at 
least,  still  shines  with  much  of  its  primeval  flre.  Mars  tyiiifies  the  middle  age  ;  Saturn 
and  Jupiter,  the  youth;  and  Uranus  and  Xeptune,  the  infancy  of  planetary  existence. 
In  the  case  of  Saturn  and  Jupiter,  we  never  see  the  real  planets,  but  only  the  outline  of 
their  atmospheres.  If  this  theory  be  true,  Jupiter  and  Saturn  now  rejiresent  the  con- 
dition in  which  our  earth  existed  ages  ago,  before  a  solid  crust  had  beeu  formed  upon  its 
surface. — (Geology,  p.  17.) 


164  THE  SOLAR  SYSTEM. 

VIII.    SATURN. 

Tie  god  of  tiire.     Sign  %  ,  an  anvient  scythe. 

Description. — We  now  reach,  in  our  outward  jour- 
ney from  the  sun,  the  most  remote  world  known  to 
the  ancients.  It  shines  with  a  steady  pale  yellow 
light,  which  distinguishes  it  from  the  fixed  stars. 
Its  orbit  is  so  vast  that  its  movement  among  the 
constellations  may  be  easily  traced  through  one's 
lifetime.  It  requires  two  and  a  half  years  to  pass 
through  a  single  sign  of  the  zodiac  ;*  hence,  when 
once  known,  it  may  be  readily  found  again.  The 
earth  leaves  it  at  conjunction,  makes  a  yearly  rev- 
olution about  the  sun,  comes  to  its  starting  point, 
and  overtakes  Saturn  in  about  thirteen  days  there- 
after.! It  is  smaller  than  Jupiter,  but  more  gor- 
geously attended.  Besides  a  retinue  of  eight  satel- 
lites, it  is  surrounded  by  a  system  of  rings,  some 
shining  with  a  golden  light,  and  others  transparept, 
— a  spectacle  as  wonderful  as  it  is  unique. 

Motion  in  Space. — Saturn  revolves  about  the  sun 
at  a  mean  distance  of  nearly  886,000,000  miles.  The 
eccentricity  of  its  orbit  is  a  trifle  more  than  that  of 
Jupiter,  so  that  while  it  may,  at  perihelion,  come 
fifty  million  miles  nearer  than  its  mean  distance,  at 
aphelion  it  swings  off  as  much  beyond.  We  can 
form  some  estimate  of  the  size  of  its  immense  orbit, 
when  we  remember  that  it  is  moving  22,000  miles 

*  Because  of  its  slow,  dreary  pace,  Saturn  was  chosen  by  the  ancients  as  the  symbol 
for  lead. 

t  From  this  the  year  of  Saturn  may  be  determined.  As  13  :  378  days  :  :  Earth's 
year  :  Saturn's  year  =  30  yr.  nearly. 


SATURN. 


165 


Salum. 


per  hour,   and  yet,   from  night  to  night,   we  can 

scarcely  detect  any  rig.  wi 

change    of    place. 

The  Saturnian  year 

is  equal    to  about 

thirty  of  ours,  and 

comprises  nearly 

25,000       Saturnian 

days,    each    about 

10;^  hours  long. 

The  Distance 
from  the  Earth  is 
found  in  the  same 
manner  as  that  of  the  other  superior  planets,  being 
least  in  opposition  and  greatest  in  conjunction.  Ac- 
cording as  the  earth  and  Saturn  occupy  different 
portions  of  their  orbits,  the  distances  between  them 
at  different  times  may  vary  nearly  300,000,000  miles. 

Dimensions.  —  The  diameter  of  Saturn  is  about 
73,000  miles.  Its  volume  is  700  times  that  of  the 
earth.  Its  density  is  about  f  that  of  water,  or  a 
little  more  than  that  of  pine  wood.  The  Saturnian 
force  of  gravity  is  therefore  scarcely  greater  than 
the  terrestrial,  so  that  a  stone  would  fall  toward 
the  surface  of  that  immense  globe  only  about  seven- 
teen feet  the  first  second. 

Seasons, — The  light  and  heat  of  the  sun  at  Saturn 
are  only  .-jJjt  that  which  we  receive.  The  axis  of 
Saturn  is  inclined  from  a  perpendicular  to  the  plane 
of  its  orbit  about  3V.*    The  seasons  therefore  are 


*  Proctor  says  26°  ;  others,  28°.     Let  the  pupil  adapt  the  paragraph  to  each  of  tliese 
estimates. 


166  THE   SOLAR  SYSTEM. 

similar  to  those  of  the  earth,  but  on  a  larger  scale. 
The  sun  climbs  in  summer  about  8°  higher  above  the 
horizon,  and  sinks  corresponding!}^  lower  in  winter. 
The  tropics  are  16°  further  apart,  and  the  arctic  and 
antarctic  circles  8°  further  from  the  poles.  Each  of 
Saturn's  seasons  lasts  more  than  seven  of  our  years. 
There  is  an  interval  of  fifteen  years  between  the 
autumn  and  spring  equinoxes,  and  between  the  sum- 
mer and  winter  solstices.  For  fifteen  years,  the  sun 
shines  On  the  north  pole,  and  a  night  of  the  same 
length  envelopes  the  south  pole.  The  atmosphere  is 
doubtless  very  dense,  as  the  belts  seem  to  indicate. 

Telescopic  Features.  —  Saturn's  Rings.  —  Galileo 
first  noticed  something  peculiar  in  the  shape  of 
Saturn.  Through  his  imperfect  telescope  it  seemed 
to  have  on  each  side  a  small  planet,  like  a  supporter, 
to  help  old  Saturn  on  his  way.  Galileo  therefore  an- 
nounced to  his  friend  Kepler  the  curious  discovery, 
that  ''  Saturn  is  threefold."  As  the  planet,  however, 
approached  its  equinoxes,  these  attendants  vanished 
from  his  instrument.  This  was  a  great  perplexity  to 
the  philosopher,  and  he  never  solved  the  mystery. 
When  the  rings  were  afterward  seen,  their  real 
form  was  not  known.  They  were  supposed  to  be  a 
kind  of  handle  attached  to  the  planet. 

Description  of  the  Rings. — The  series  consists  of 
three  rings  of  unequal  breadth,  surrounding  the 
planet  at  the  equator.  The  exterior  ring  is  separated 
from  the  middle  one  by  a  distinct  break,  while  the 
interior  ring  seems  joined  to  the  middle  one.  They 
differ  in  their  brightness ;  the  exterior  ring  is  of  a 
grayish  tint ;  the  middle  one  is  the  most  brilliant^ 


SATURN.  167 

being  more  luminous  than  Saturn  itself  ;  the  interior 
one  is  darker  and  has  a  purple  tinge.  The  two  outer 
rings  are  known  as  the  bright  rings,  and  the  inner 
one  is  called  the  dusky  ring.  The  exterior  and  mid- 
dle rings  are  both  opaque  and  cast  on  the  planet  a 
distinct  shadow  ;  while  the  interior  one  is  so  trans- 
parent that  it  appears  upon  the  globe  of  Saturn  as  a 
dark  band  through  which  the  surface  of  the  planet 
is  readily  seen. 

Saturn's  Rings.     (Proctor.) 

Miles. 

Diameter  of  exterior  ring 166,920 

Breadth  of  exterior  ring 10,000 

Diameter  of  middle  ring 144,300 

Breadtli  of  middle  ring 17,000 

Distance  between  exterior  and  middle  ring 1,700 

Diameter  of  interior  ring 92,000 

Breadth  of  interior  ring 8,600 

Distance  of  interior  ring  from  the  pljftiet 10,000 

Entire  breadth  of  ring  system 37,570 

Thickness  of  rings,  less  than 100 

Rotation.— T[\Q  rings  revolve  around  Saturn  in 
about  10|  hours,  in  the  same  direction  as  the  planet 
rotates  on  its  axis.  The  globe  of  Saturn  is  not 
exactly  at  the  center  of  the  rings.  This  fact,  com- 
bined with  the  rotary  motion,  is  essential  to  the 
stability  of  the  rings,  preventing  them  from  being 
precipitated  upon  the  planet. 

Phases  of  the  Rings. — The  plane  of  the  rings  is 
inclined  about  28°  to  the  ecliptic.  In  its  revolution 
about  the  sun,  the  axis  of  Saturn  remaining  parallel 
to  itself,  the  sun  sometimes  illumines  the  northern 
and  sometimes  the  southern  face  of  the  rings.  At 
JSaturn's  equinoxes,  only  the  edge  receives  the  light, 


168 


THE  SOLAR  SYSTEM. 


and  the  rings  are  invisible  to  us,  except  with  the 
most  powerful  telescopes,  and  then  only  as  a  line  of 
light.  The  body  of  the  planet  constantly  cuts  off 
the  sun's  rays  from  a  portion  of  the  rings,  and  also 
serves  to  conceal  from  our  view  some  of  the  lumin- 


Fig.  71. 


iji.ii:,iJilJii;iiJ;:lilLi:li;.,!Jiiilljlii!iilli];!iilililliti!lil!lllli 

Phases  of  Saturn's  Rings. 


ous  part.  By  a  careful  study  of  the  cut,  these  various 
positions  of  the  planet  and  rings,  with  the  favorable 
times  for  observation,  may  be  understood. 

Composition  of  the  Rings. — It  is  now  generally 
believed  that  the  rings  consist  of  a  cloud  of  tiny 
satellites, — too  small  to  be  seen  with  the  telescope, — 
revolving  about  the  planet  (see  Nebular  Hypoth- 
esis). 

JBelts.— The  surface  of  Saturn  is  traversed  by  f^int 


SATtJRM. 


UQ 


dusky  belts  of  a  fstr  less  distinct  and  definite  appear- 
ance than  those  upon  Jupiter.  The  equatorial  re- 
gions are  more  strongly  marked  than  the  other  parts 
of  the  disk. 

Composition  of  the  Planet.— It  is  quite  probable 
that  Saturn,  like  Jupiter,  has  no  solid  crust,  but  con- 
sists of  molten  matter  surrounded  by  vapor  that  con- 
tinually rises  from  the  heated  interior  (note,  p.  163). 

Satellites. — Saturn  has  eight  satellites. 


Names  of  Saturn's 
fr'atellites. 


I I  Jliinas 

II i   Eiu'elaau.s., 


III.  . 
IV... 
V. . .  . 
VI... 
VII.. 
VIII. 


Tethys. .. 

Dioiie 

Rliea  .... 

Tit^ui .... 
Hyperion 
Japetus. . 


Distance  from 

t<aturn  in 

miles. 


120,800 
15.5,015 
191,248 
245,87(5 
343,414 
796,157 
1,006,656 
2,313,835 


.\p))ro,\imate 

diameter  in 

miles. 


1,000 

•> 

500 

500 

1,200 

3,300 

? 

1,800 


fiidereal 

Period  in 

days. 


0.94 
1.37 
1.88 
2.73 
4.51 
16.94 
21.29 
79. to 


Titan  is  the  largest,  and  in  size  exceeds  Mer- 
cury. Enceladus  and  Mimas  are  the  faintest  of 
twinklers,  and  can  be  seen  only  with  a  powerful 
telescope.  They  were  first  detected  by  Herschel, 
''threading  like  pearls  the  silver  line  of  light,"  to 
which  the  ring,  then  seen  edgewise,  was  reduced, — 
advancing  off  it  at  either  end,  returning,  and  then 
hiding  themselves  behind  the  planet.  The  first 
three  of  these  moons  are  nearer  to  Saturn  than  our 
moon  is  to  the  earth,  but  Japetus  is  nearly  ten  times 
as  distant :  so  that  the  diameter  of  the  Saturnian 
system  is  nearly  four  and  a  half  million  miles. 

Saturnian    Scenery.  —  The    magnificence    of    the 


170 


THE  SOLAR  SYSTEM. 


scenery  upon  Saturn  must  surpass  anything  with 
which  we  are  famihar.  In  the  cut,  is  given  an  ideal 
view  of  a  landscape  located  upon  the  planet  at  a  lati- 
tude of  about  28°,  taken  at  midniglit.  The  rings  form 
an  immense  arch,  which  spans  the  sky  and  sheds  a 

Fig.  m. 


Ideal  Landscape  on  Saturn,  supposing  a  solid  crust  to  exist. 

soft  radiance  around ;  while,  to  add  to  the  strange 
beauty  of  the  night,  eight  moons  in  all  their  different 
phases — full,  new,  crescent,  or  gibbous — light  up  the 
starry  vault. 


IX.    URANUS. 

"  Heaven,"  the  most  ancient  of  the  gotls.    Sign,  IJl ;  H,  the  initial  letter  of  Hersehel, 
with  a  planet  suspended  from  the  cross-bar. 

Description. — On  the  13th  of  March,  1781,  between 
10  and  11  p.  M.,  Sir  William  Hersehel  was  examining 


URANUS.  171 

with  his  great  telescope  some  stars  in  the  constella- 
tion Gemini.     A  small  star  attracting  his  attention, 
he  observed  it  with  a  higher  magnifying    power, 
when,    unlike   the    fixed    stars,    its    disk   widened. 
Watching  it  for  several  nights,  he  detected  its  mo- 
tion in  space ;  but,  mistaking  its  true  character,  he 
V    announced  the  discovery  of  a  new  comet.     A  few- 
months  examination  revealed  the  error,  and  the  new 
^  body — new  to  us,  but  older  perhaps   than  our  own 
^  world* — was  admitted  to  be  a  member  of  the  solar 
system. 

Uranus  may  be  seen  in  a  dark  sky,  by  a  person  of 
strong  eyesight,  if  he  previously  knows  its  exact 
position  among  the  stars.  Its  faintness  is  due  to  its 
great.,  distance  from  the  earth.  Were  it  as  near  as 
the  sun,  it  would  appear  twice  as  large  as  Jupiter. 

Motion  in  Space. — Uranus  revolves  about  the  sun 
at  a  mean  distance  of  nearly  1,782,000,000  miles.  Its 
year  exceeds  eighty-four  of  ours. 

Dimensions. — Its  diameter  is  about  33,000  miles. 
Its  density  is  about  equal  to  that  of  the  water  from 
the  Dead  Sea.  The  force  of  gravity  upon  the  surface 
of  the  planet  is  ^^  that  upon  the  earth. 

Seasons. — We  know  little  of  the  seasons  of  Uranus. 
If  its  axis  lies  in  the  plane  of  its  orbit,  the  sun 
must  wind  in  a  spiral  form  around  the  planet.  The 
light  and  heat  are  less  than  x.^xttt  of  that  which  we 

*  It  is  now  known  that  Uranus  had  been  previously  observed  by  other  astronomers. 
Le  Monier  at  Paris  had  watched  it  for  twelve  successive  nights,  but  pronounced  it  a 
fixed  star.  He  had  also  seen  it  on  previous  occasions,  and  had  he  been  an  orderly  ob- 
server, he  would  doubtless  have  detected  its  planetary  character  ;  but  he  was  extremely 
careless,  as  may  be  inferred  from  the  fact  related  by  Arago,  that  he  had  been  shown  one 
of  Le  Monier's  observations  of  this  planet  written  on  a  paper  bag  which  originally  con- 
taiued  hair-powder  puivliased  at  a  perfumer's. 


l7^  THE  SOLAR  SYSTEM. 

receive;  the  light  has  been  estimated  to  be  about  the 
quantity  that  would  be  afforded  by  three  hundred 
full  moons.  The  inhabitants  of  Uranus,  if  any  such 
exist,  can  see  Saturn,  and  perhaps  Jupiter,  but  none 
of  the  planets  \vithin  the  orbit  of  the  latter. 

Telescopic  Features. — Xo  spots  or  belts  have  been 
discovered.  The  time  of  rotation  and  the  other 
features  so  familiar  to  us  in  the  nearer  planets  are 
therefore  unknown  with  regard  to  Uranus. 

Satellites. — Uranus  has  four  moons,  of  which 
little  is  known  except  the  curious  fact  that  their 
orbits  are  nearly  perpendicular  to  the  plane  of  the 
planet's  orbit,  and  that  their  movements  are  appar- 
ently retrograde — /.  e.,  in  the  same  direction  as  the 
hands  of  a  watch. 


X.    NEPTUNE. 

The  god  of  the  sea.    Sign,  "f  ,  his  trident. 

Description- — Xeptune  is  the  far-off  sentinel  at  the 
outpost  of  the  solar  system,  being  the  most  distant 
planet  of  which  we  have  any  knowledge.  It  is  in- 
visible to  the  naked  eye,  and  appears  in  the  telescope 
as  a  star  of  the  sixth  magnitude. 

Discovery. — For  many  years,  the  motions  of  Ura- 
nus had  been  such  as  to  baffle  the  most  perfect  calcula- 
tions. While  far-distant  Saturn,  after  his  journey  of 
thirty  years,  came  around  to  his  place  true  to  the  min- 
ute, Uranus  defied  arithmetic,  and  refused  to  con- 
form to  the  time  set  down  for  him  on  the  heavenly 
dial. 


KEPTUNjbi.  173 

At  length  it  was  suggested  that  there  was  another 
planet  exterior  to  Uranus,  whose  attraction  produced 
these  perturbations.  So  marked  was  this  impression 
with  Herschel,  that  he  writes  :  "  We  see  it  as  Colum- 
bus saw  America  from  the  shores  of  Spain.  Its 
movements  have  been  felt  trembling  along  the  far- 
reaching  line  of  our  analysis  with  a  certainty  not 
far  inferior  to  ocular  demonstration." 

Finally,  two  young  mathematicians,  Leverrier,  of 
Paris,  and  Adams,  of  Cambridge,  England,  each  un- 
known to  the  other,  set  about  the  task  of  finding  the 
place  of  this  new  planet.  The  problem  was  this  : 
Given  the  disturbances  produced  by  the  attraction  oj 
the  unknown  planet,  to  find  its  orbit  and  its  place  in 
the  orbit. 

Adams,  after  assiduous  labor  for  nearly  two  years, 
completed  his  calculations  and  submitted  them  to 
Prof.  Airy,  the  Astronomer  Royal,  in  1845.  In  the 
summer  of  184G,  Leverrier  laid  a  paper  before  the 
Academy  of  Sciences  in  Paris,  announcing  the 
position  of  the  unknown  planet.  Prof.  Airy,  hear- 
ing of  this,  was  so  impressed  with  the  value  of 
Adams's  calculations,  that  he  wrote  to  Prof.  Challis, 
of  Cambridge,  to  search  that  quarter  of  the  heavens. 
Prof.  Challis  did  as  requested,  and  saw  a  star  which 
afterward  proved  to  be  the  planet  so  anxiously 
sought  for,  although  at  that  time  he  failed  to  ascer- 
tain its  true  character.  In  September,  of  the  same 
year,  Leverrier  wrote  to  Berlin,  asking  for  assistance 
in  searching  for  the  planet.  Dr.  Galle,  on  receiv- 
ing the  request,  turned  the  large  telescope  of  the 
Observatory  to  the  place  indicated,  and  almost  im- 


IH  THE  SOLAR   SYSTEM. 

mediately  detected  a  bright  star  not  laid  down  in  the 
maps.  This  proved  to  be  the  predicted  planet,  found 
within  less  than  a  degree  of  the  spot  described  by 
Leverrier. 

Such  is  the  history  of  one  of  the  grandest  achieve- 
ments of  the  human  mind.  It  stands  as  an  ever 
fresh  and  assuring  proof  of  the  exactness  of  astro- 
nomical calculations,  and  the  power  of  the  intellect 
to  understand  the  laws  of  the  God  of  Nature. 

Motion  in  Space. — Neptune  revolves  about  the  sun 
at  a  mean  distance  of  about  2,790,000,000  of  miles. 
The  Neptunian  year  is  equal  to  nearly  165  terrestrial 
ones.  Its  motion  in  its  orbit  is  the  slowest  of  any  of 
the  planets,  since  it  is  the  most  remote  from  the  sun. 
The  velocity  decreases  from  Mercury,  which  moves 
at  the  rate  of  about  105,000  miles  per  hour,  to  Nep- 
tune, whose  rate  is  only  12,000  miles. 

Dimensions. — Neptune's  diameter  is  about  37,000 
miles.  Its  volume  is  nearly  100  times  that  of  the 
earth.   Its  density  is  a  little  less  than  that  of  Uranus. 

Seasons. — As  the  inclination  of  its  axis  is  un- 
known, nothing  can  be  ascertained  concerning  its 
seasons.  The  sun  gives  to  Neptune  but  xio^  the 
light  and  heat  which  we  receive. 

Though  Neptune  is  at  the  extreme  of  the  solar 
system,  2,790,000,000  miles  beyond  us,  the  same 
heavens  bend  above,  the  Milky  Way  is  no  nearer  to 
the  eye,  and  the  fixed  stars  shine  no  more  brightly. 
The  planets,  however,  are  all  too  near  the  sun  to  be 
seen,  except  Saturn  and  Uranus.  The  Neptunian 
astronomers,  if  there  be  any,  are  well  situated  for 
measuring  the  annual  parallax  of  the   stars,  since 


METEOfiS  AND  SHOOTING  STaHS.  175 

NeptuDe  has  an  orbit  of  5,580,000,000  miles  in 
diameter,  and  hence  the  angle  must  be  thirty  times 
as  great  as  that  which  the  terrestrial  orbit  affords. 

Telescopic  Features. — On  account  of  the  recent 
discovery  of  this  planet  and  its  immense  distance, 
nothing  is  known  of  its  rotation  or  physical  features. 

Satellites. — Neptune  has  one  moon,  at  nearly  the 
same  distance  from  it  as  our  own  moon  is  from  the 
earth.  The  revolution  of  this  body  about  the  planet, 
which  is  accomplished  in  about  six  days,  has  fur- 
nished the  materials  for  calculating  the  mass  of 
Neptune. 


III.     METEORS      AND      SHOOTING 
STARS. 

Description. — All  are  familiar  with  those  luminous 
bodies  that  flash  through  our  atmosphere  as  if  the 
stars  were  indeed  falling  from  heaven.  Different 
names  have  been  applied  to  them,  although  the  dis- 
tinction is  not  very  definite. 

(1)  Aerolites  are  those  stony  or  iron  masses 
which  descend  to  the  earth. 

(2)  Meteors  are  luminous  bodies  which  have  a 
sensible  diameter  and  a  spherical  form.  They  fre- 
quently pass  over  a  great  extent  of  country,  and  are 
seen  for  some  seconds.  Many  leave  behind  them  a 
train  of  glowing  sparks  ;  others  explode  with  reports 
like  the  discharge  of  artillery,— the  pieces  either 
continuing  their  course,  or  falling  to  the  earth  as 


176  THE  SOLAR  SYSTEM. 

aerolites.     Some  meteors  pass  on  into  space ;  some 
are  vaporized  ;  while  others  are  burned,  and  the  ashes 
and  fragments  fall  to  the  ground. 
(3)  Shooting  Stars  are  those  evanescent,  brilliant 


Fig.  73. 


A  Meteor  vriih  its  Train. 


points  that  suddenly  dart  through  the  higher  regions 
of  the  air,  leaving  a  fiery  train  behind. 
1.  Aerolites. — The  fall  of  aerolites  is  frequently  men- 


METEORS  AND  SHOOTING  STARS.  177 

tioned  and  well  authenticated.  Chinese  records  tell 
of  one  as  long  ago  as  616  B.C.,  that,  in  its  fall,  broke 
several  chariots  and  killed  ten  men.  A  block  of 
stone,  equal  to  a  full  wagon-load,  fell  in  the  Helles- 
pont, B.C.  465.  By  the  ancients,  these  stones  were 
held  in  great  repute.  The  Emperor  Jehangire,  it  is 
related,  had  a  sword  forged  from  a  mass  of  meteoric 
iron  which  fell  in  the  Punjab  in  1620.  In  1795,  amass 
was  seen,  by  a  ploughman,  to  descend  not  far  from 
where  he  was  standing.  It  threw  up  the  soil  on 
every  side,  and  penetrated  some  distance  into  the 
solid  rock  beneath.  In  1807,  there  was  a  shower  of 
stones,  one  weighing  200  lbs.,  at  Weston,  Connect- 
icut. A  mass  once  fell  in  South  America,  that  was 
estimated  to  weigh  fifteen  tons.  When  first  dis- 
covered, it  was  so  hot  as  to  prevent  all  approach. 
Upon  its  cooling,  many  efforts  were  made,  by  some 
travelers  who  were  present,  to  detach  specimens, 
but  its  hardness  was  too  great  for  the  tools  that  they 
possessed.  In  Yale  College  cabinet,  there  is  a  mass 
of  meteoric  iron,  weighing  1,635  lbs. 

Aerolites  consist  of  elements  which  are  famil- 
iar. The  analysis  of  these  stellar  objects  gives  us 
names  as  commonplace  as  if  they  had  known  a  far 
less  romantic  origin, — iron,  tin,  copper,  nickel, 
cobalt,  lime,  magnesia,  oxygen,  sulphur,  phos- 
phorus ;  in  all,  about  twenty  elements  have  been 
found.  This  fact  is  interesting  as  revealing  some- 
thing of  the  chemistry  of  the  region  of  space,  concern- 
ing which  we  otherwise  know  little.  The  compounds, 
however,  are  so  peculiar  as  to  distinguish  an  aerolite 
ITOQi  other  substances.     For  example,  meteoric  iron, 


178 


THE  SOLAK   SYSTEM. 


a  prominent  constituent  of  aerolites,  is  an  alloy  that 
has  never  been  found  in  terrestrial  minerals. 

2.  Meteors. — The  records  of  meteors  are  even  more 
wonderful  than  those  of  aerolites.  It  is  related  that 
at  Crema,  Italy,  one  day  in  the  loth  century,  the  sky 
at  noonday  became  dark, — a  cloud  of  appalling  black- 
ness overspreading  the  heavens.  Upon  this  cloud, 
appeared  the  semblance  of  a  great  peacock  of  fire 

Fig.  7i. 


Copy  of  a  Print  Showing  the  Peculiar  Crystalline  Sti~ucture  of  Meteoric  Iron. 

flying  over  the  town.  This  suddenly  changed  to  a 
huge  pyramid,  that  rapidly  traversed  the  sky. 
Thence  arose  awful  lightnings  and  thunderings, 
amid  which  there  fell  upon  the  plain  rocks,  some 
of  which  weighed  100  lbs.  In  1803,  a  brilliant  fire- 
ball was  seen  traversing  Normandy  with  great 
velocity,  and  some  moments  after,  frightful  explo- 
sions, like  the  noise  of  cannon,  were  heard  coming 
from  a  black  cloud  hanging  in  the  clear  sky;  they 


METEORS  AND  SHOOTING  STARS.  179 

were  prolonged  for  five  or  six  minutes.  These  dis- 
charges were  followed  by  a  shower  of  heated  stones, 
some  weighing  over  24  lbs.  In  1819,  a  meteor  was 
witnessed  in  Massacb.usetts  and  Maryland,  the 
diameter  of  which  was  estimated  at  half  a  mile.  In 
July,  1860,  a  brilliant  fireball  passed  over  the  State 
of  New  York,  from  west  to  east,  and  was  last  seen 
far  out  at  sea.  On  the  evening  of  Feb.  12th,  1875,  a 
magnificent  meteor  "  illumined  the  entire  State  of 
Iowa,  and  parts  of  Missouri,  Illinois,  Wisconsin,  and 
Minnesota.  The  aerolites  that  have  been  collected 
show  its  weight  to  have  been  fully  5,000  lbs." 

3.  Shooting  Stars. — One  of  the  earliest  accounts  of 
star-showers  is  that  which  relates  how,  in  472,  the 
sky  at  Constantinople  appeared  to  be  alive  with  fly- 
ing stars  and  meteors.  In  some  Eastern  annals  we 
are  told  that  in  October,  1202,  "the  stars  appeared 
like  waves  upon  the  sky.  They  flew  about  like 
grasshoppers,  and  were  dispersed  from  left  to  right." 
It  is  recorded  that  in  the  time  of  King  William  II. 
there  occurred  in  England  a  wonderful  shower  of 
stars,  which  "seemed  to  fall  like  rain  from  heaven. 
An  eye-witness,  seeing  where  an  aerolite  fell,  cast 
water  upon  it,  which  was  raised  in  steam,  with  a 
great  noise  of  boiling."  * 

Showers  of  1799  and  1833.— The  most  remark- 
able accounts  are  those  of  the  showers  of  November 
12th,  1799,  and  November  13th,  1833.  Humboldt,  in 
describing   the   former,  says  the   sky  was   covered 


*  Rastel  says  concerning  it :  "  By  the  report  of  the  common  people  in  this  kynge's 
time,  diverse  great  wonders  were  seene,  and  tlierefore  the  kynge  was  told  by  diverse  pjf 
his  familiars  that  God  was  not  content  with  his  lyvyng." 


180  THE  SOLAR   SYSTEM. 

with  innumerable  fiery  trails,  which  incessantly 
traversed  the  sky.  From  the  beginning  of  the  phenom- 
enon, there  was  not  a  space  in  the  heavens  three 
times  the  diameter  of  the  moon  that  was  not  filled 
every  instant  with  the  celestial  fireworks, — large 
meteors  blending  constantly  their  dazzling  brilliancy 
with  the  long  phosphorescent  paths  of  the  shooting 
stars.     (See  notes,  p.  305.) 

The  latter  shower  was  most  brilliant  on  this  conti- 
nent, and  was  visible  from  the  lakes  to  the  equator. 
Phosphoric  lines  swept  over  the  sky  like  the  flakes 
of  a  snow-storm.  Large  meteors  darted  across  the 
heavens,  leaving  luminous  trains  behind  them  that 
were  visible  sometimes  for  half  an  hour :  they  gen- 
erally shed  a  soft  white  light :  occasionally,  how- 
ever, yellow,  green,  and  other  colors  varied  the 
scene.  Irregular  fireballs,  almost  stationary,  glared 
in  the  sky ;  one  especially,  larger  than  the  moon, 
hung  in  mid-air  over  Niagara  Falls,  and  mingled  its 
light  with  the  foam  and  mist  of  the  cataract.  In 
many  sections,  the  people  were  terror-stricken  by 
the  awful  spectacle,  and  supposed  that  the  end  of 
the  world  had  come. 

Inferior  showers  were  seen  in  1831,  and  1832,  and 
in  the  succeeding  years,  until  1839.  These  did  not 
compare  in  brilliancy  with  the  remarkable  phenom- 
enon of  1833.  There  was  an  interval  of  about  3-4 
years  between  the  great  showers  of  1799  and  1833 ;  this 
seemed  to  indicate  another  shower  in  1866  or  1867. 

In  November,  1866,  the  people  of  both  hemispheres 
were  literally  awake  to  the  subject.  Newspapers 
aroused  the  mpst  sluggish  imagination  with  thrilling 


METEORS   AND    SHOOTING   STARS.  181 

accounts  of  the  scenes  presented  in  1790  and  1833. 
Extempore  observatories  were  established  at  every 
convenient  point.  Watchmen  were  stationed,  and 
the  city  bells  were  to  be  rung  on  the  appearance  of 
the  first  wandering  celestial  visitor.  The  exact 
night  was  not  definitely  known,  but,  for  fear  of  a 
mistake,  the  11th,  12th,  and  13th  were  generally  ob- 
served. The  anxious  vigils,  the  fruitless  scannings 
of  the  sky,  the  disappointment,  the  meteors  that 
were  dimly  thought  to  be  seen, — all  these  were  re- 
corded in  the  memory  of  the  temporary  astronomers 
of  that  year. 

While,  however,  the  people  of  America  were  thus 
disappointed,  there  was  enacted  in  England  a  dis- 
play brilliant  indeed,  though  inferior  to  the  one  of 
1833.  The  staff  at  Greenwich  Observatory  counted 
about  8,000  meteors. 

In  November,  1867,  the  long-expected  shower  was 
seen  in  this  country,  but  it  failed  to  satisfy  the  pub- 
lic anticipation.  The  sky  was,  however,  illumined 
with  shooting  stars  and  meteers,  some  of  which  ex- 
ceeded Jupiter  or  Venus  in  brilliancy. 

Number  of  Meteors  and  Shooting  Stars. — Prof, 
Newton  estimates  that  the  average  number  of  me- 
teors that  traverse  the  atmosphere  daily,  and  which 
are  large  enough  to  be  visible  to  the  eye  on  a  dark, 
clear  night,  is  7,500,000  ;  and  if  to  these  the  tele- 
scopic meteors  be  added,  the  number  would  be  in- 
creased to  400,000,000.  In  the  space  traversed  by 
the  earth,  there  are,  on  the  average,  in  each  volume 
the  size  of  our  globe  (including  its  atmosphere),  as 
many  as  13,000  small  bodies,  each  one  capable  of 


182  THE   SOLAH   SYSTEM. 

furnishing  a  shooting  star  visible  under  favorable 
circumstances  to  the  naked  eye. 

Annual  Periodicity  of  the  Star-Showers. — On  al- 
most any  clear  night,,  from  five  to  seven  shooting 
stars  may  be  seen  per  hour,  but  in  certain  months 
they  are  much  more  abundant.  Arago  names  the 
following  principal  dates  : 

April  4-11  ;  17-25.  October  (about)  15. 

August  9-11.  November  13-14. 

Origin. — Aerolites,  meteors,  and  falling  stars  are 
produced  by  small  bodies — planets  in  miniature — 
revolving,  like  our  earth,  about  the  sun.  Their  or- 
bits intersect  the  orbit  of  the  earth,  and  if,  at  any 
time,  they  reach  the  point  of  crossing  exactly  with 
the  earth,  there  is  a  collision.  Their  mass  is  so 
small,  that  the  earth  is  not  jarred  any  more  than 
a  railway  train  would  be  by  a  pebble  thrown 
against  it. 

These  small  bodies  may  come  near  the  earth  and 
be  drawn  to  its  surface  by  the  power  of  attraction  ; 
or  they  may  sweep  through  the  higher  regions  of 
the  atmosphere,  and  then  escape  its  grasp :  or, 
finally,  they  may,  under  certain  conditions,  be  com- 
pelled to  revolve  many  times  around  the  earth  as 
satellites. 

The  November  "  meteoroids  "  (as  these  bodies  are 
called  before  igniting)  move  at  the  rate  of  26  miles 
per  second  in  a  direction  nearly  opposite  that  of  the 
earth.  They,  therefore,  meet  our  atmosphere  with  a 
relative  velocity  of  44  miles  per  second.  As  they 
sweep  through  the  air,  the  friction  partly  arrests 


METEORS  AND  SHOOTING  STARS.  183 

their  motion,  and  converts  it  into  heat  and  light. 
The  body  thus  becomes  visible  to  us.  Its  size  and 
direction  determine  its  appearance.  If  very  small, 
it  is  consumed  in  the  upper  regions,  and  leaves  only 
the  luminous  trail  of  a  shooting  star.  If  of  large 
size,  it  may  sweep  along  at  a  high  elevation,  or 
plunge  directly  toward  the  ground.  Becoming 
highly  heated  in  its  course,  it  sheds  a  vivid  light, 
while,  unequally  expanding,  it  explodes,  throwing 
off  large  fragments  which  fall  to  the  earth  as 
aerolites,  or  continue  their  separate  course  as  me- 
teors. The  cinders  of  the  consumed  portion  rain 
down  on  us  as  fine  meteoric  dust.  * 

Meteoric  Rings. — These  little  bodies,  it  is  thought, 
do  not  generally  revolve  individually  about  the  sun, 
but  myriads  of  them  are  collected  in  a  ring. 
"When  the  earth  passes  through  one  of  these  floating 
girdles,  a  star-shower  follows.  This  would  account 
for  their  regular  appearance  at  certain  seasons  of 
the  year.  The  November  meteoroids  are  not,  like 
the  August  ones,  uniformly  distributed  through  the 
ring,  but  are  principally  collected  in  a  swarm  that 
has  a  period  of  33^  years  ;  hence  the  August  shower 
occurs  quite  regularly  each  summer,  while  the  great 
November  one  happens  only  three  times  in  a  cen- 
tury. The  orbit  of  the  November  stream  extends 
beyond  that  of  Uranus.  The  point  where  it  crosses 
the  earth's  orbit  moves  forward  about  50"  per  annum, 
and  thus  that  star-shower  occurs  about  a  day  later 
at  each  return.     It  takes  three  or  four  years  for  this 

♦  Prof.  Young  estimates  that  100  tons  of  meteoric  matter  fall  upon  the  earth  daily 
from  outer  space. 


184 


THE  SOLAR  SYSTEM. 


Fig.  75. 


swarm  to  pass  the  node,  showing  that  the  shoal  of 
meteoroids  occupies  about  -^  of  its  orbit.     The  earth 

in  its  annual  revolution  about 
the  sun  is  supposed  to  en- 
counter several  hundred  of 
these  meteoric  rings. 

The  Physical  Relation  be- 
tween meteoroids  and  comets 
is  now  generally  acknowl- 
edged. The  orbit  of  the  Au- 
gust meteors  is  known  to  be 
identical  with  Comet  III  1862 
(Swift's),  and  that  of  the 
November  14th  shower  cor- 
responds with  Comet  I  1866 
(Tempel's).  The  small  show- 
ers of  November  24  and  27 
are  thought  to  be  produced 
by  meteors  traveling  in  the 
path  of  the  two  dissevered 
parts  of  Biela's  comet. 

The  grand  problem  of  me- 
teoric astronomy  to-day  is  to 
identify  the  numerous  mete- 
oric rings,  and  to  detect  their 
allied  comets.  Being  thus 
intimately  associated,  they 
must  have  a  common  history. 
Prof.  Newton,  the  great  ad- 
n  7, ,  r,*  .     .  ,r .  vocate  of  this  theory,  broadly 

Orhit  of  the  August  Meteors.  •'  '  -^ 

asserts  that  every  meteoric 
stone  was  once  a  part  of  a  comet,  and  every  mete- 


COMETS.  185 

oric  shower  consists  of  broken  fragments  of  some 
known  or  unknown  comet. 

Radiant  Point. — The  meteoroids  are,  of  course, 
moving  in  parallel  lines,  but,  by  an  optical  illusion, 
they  seem  to  radiate  in  all  directions,  the  radiant 
point  being  in  that  part  of  the  heavens  which  the 
earth  is  then  approaching.  *  A  star  (m)  in  the  blade 
of  the  sickle  is  the  point  from  which  the  stars  in  the 
November  shower  radiate,  while  one  in  Perseus  (y) 
is  the  radiant  point  of  the  August  shower. 

Height. — Herschel  estimates  the  average  height  of 
shooting  stars  above  the  earth  to  be  seventy-three 
miles  at  their  appearance,  and  fifty -two  at  their 
disappearance. 

Weight.— Prof.  Harkness  calculates  that  the  aver- 
age weight  of  shooting  stars  does  not  differ  much 
from  one  grain. 


IV.    COMETS. 

We  come  now  to  notice  a  class  of  bodies  the  most 
fascinating,  perhaps,  of  any  in  astronomy.  The 
suddenness  with  which  comets  flame  out  in  the 
sky,  the  enormous  dimensions  of  their  fiery  trains, 
the  swiftness  of  their  flight,  the  strange  and  mys- 
terious forms  they  assume,  their  departure  as  un- 
heralded as  their  advent, — all  seem  to  bid  defiance 
to  law,  and  partake  of  the  marvellous.   Superstitious 

*  The  same  illusion  is  seen  if,  looking  upward,  we  watch  snow-flakes  falling  during  a 
calm.  Those  coming  directly  toward  our  eyes  seem  to  be  motionless,  and  the  rest  to 
separate  from  them  in  diverging  lines.  This  is  the  effect  of  perspective,  and  the  "  rad- 
iant point"  is  really  the  "vanishing  point"  of  the  parallel  lines  through  which  the 
meteors  are  moving.    See  Newcomb's  Astronomy,  p.  399. 


186  THE  SOLAR  SYSTEM. 

fears  have  been  excited  by  their  appearance,  and 
they  have  been  looked  upon  in  every  age  as 

"  Threatening  the  world  with  famine,  plague,  and  war ; 
To  princes,  death  ;  to  kingdoms,  many  curses ; 
To  all  estates,  inentable  losses  ; 
To  herdsmen,  rot ;  to  plowmen,  hapless  seasons; 
To  sailors,  storms  ;  to  cities,  civil  treasons."  • 

Description. — The  term  comet  signvQ.es  a.hairy  body. 
A  comet  consists  usually  of  three  parts  ; — the  nucleus, 
a  bright  point  in  the  center  of  the  head ;  the  cotna 
(hair),  the  cloud-like  mass  surrounding  the  nucleus  ; 

Fig.  76.  Fig.  77. 


Comet  Kithout  a  Nufleus.  Comet  mth  a  Nitdeus. 

and  the  tail,  a  luminous  train  extending  generally  in 
a  direction  opposite  to  the  sun.  There  ai-e  comets 
without  the  tail,  and  others  with  several  tails,  while 
some  are  deprived  of  even  the  nucleus.  The  last 
consist  merely  of  a  fleecy  mass,  known  to  be  a 
comet  from  its  orbit  and  rapid  motion. 

•  Thas  the  comet  of  43  b.  c,  which  appeared  just  after  the  assassination  of  Julias 
CiBsar,  was  looked  upon  by  the  Romans  as  a  celestial  chariot  sent  to  convey  his  soul 
heavenward.  An  old  English  writer  observes :  "  Cometes  signifle  corruptions  of  the 
a\Te.  Tliey  are  signes  of  earthquakes,  of  warres,  of  changyng  kjTigedomes,  great 
dearthe  of  com,  yea,  a  common  death  of  man  and  beast."  Another  remarks  :  "  Expert 
ence  is  an  eminent  e\'idence  that  a  comet,  like  a  sword,  portendeth  war  ;  and  a  hairy 
comet,  or  a  comet  with  a  beard,  denoteth  the  death  of  kings,  as  if  God  and  nature  in- 
tended by  comets  to  ring  the  knells  of  princes,  esteeming  bells  in  churches  upon  earth 
Dot  sacred  enough  for  such  illustrious  and  eminent  performances." 


eoMEtg.  187 

Comets  are  not  confined,  like  the  planets,  to  the 
limits  of  the  zodiac,  but  appear  in  every  quarter  of 
the  heavens,  and  move  in  every  conceivable  direc- 
tion. When  first  seen,  the  comet  resembles  a  faint 
spot  of  light  upon  the  dark  background  of  the  sky: 
as  it  approaches  the  sun  the  brightness  increases, 
and  the  tail  begins  to  show  itself.  Generally  it  is 
brightest  near  perihelion,  and  gradually  fades  away 
as  it  recedes,  until  it  is  finally  lost,  even  to  the 
telescope.* 

The  Time  of  Greatest  Brilliancy  depends  some- 
what on  the  position  of  the  earth.  If,  as  represented 
in  the  figure,  the  earth  is  at 

°  '  Fig.  78. 

a  when  the  comet,  moving  to- 
ward perihelion,  is  at  r,  the 
comet  will  appear  more  dis- 
tinct than  when  it  is  more  dis- 
tant at  P,  although  at  the  latter 
point  it  is  really  brighter.  If, 
however,    the    earth    is    at  c  omt  o/ coraet. 

at  the  time  of  perihelion,  the 

comet  will  be  much  more  conspicuous.     Again,  if 
the  earth  is  passing  from  a  to  6  during  the  time 

*  While  a  comet  remains  in  regions  beyond  the  planets,  where  the  temperature  is  be- 
low —  140°  C,  its  matter  must  be  chiefly  solid  or  liquid.  On  its  approach  to  the  sun,  its 
enveloping  atmosphere  (if  none  existed,  one  will  now  be  formed)  will  expand,  and  the 
nucleus  will  appear,  surrounded  by  a  blaze  of  light,  feeble  at  first,  but  becoming  more 
and  more  brilliant,  and  so  producing  the  head,  or  coma,  of  the  comet.  Many  comets  do 
not  go  beyond  this  first  phase,  and,  being  exposed  only  to  a  moderate  heat,  remain 
telescopic.  Others,  piercing  further  the  solar  system,  and  reaching  a  higher  tempera- 
ture, develop  a  more  abundant  atmosphere.  The  sun,  while  attracting  to  himself  the 
nucleus,  has  power  to  repel  some  of  the  matter  of  the  atmosphere ;  how  or  why,  we 
know  not.  Enough,  that  certain  parts  fly  ofl"  as  if  driven  by  a  gale,  so  making  the  tail, 
which  increases  more  and  more  until  the  atmosphere  is  exhausted.  Meanwhile,  remark- 
able changes  take  place  in  the  nucleus.  Eruptions  occur.  Pieces  are  sometimes  thrown 
off  large  enough  to  form  a  new  comet,  and  showers  of  spark-like  particles,  with  oecasion- 
ally  stony  masses,  fill  the  orbit  of  the  comet  with  meteoroids.— Sc/iiaporeUi. 


188  THE  SOLAR  SYSTEM. 

the  comet  is  near  the  sun,  it  will  appear  less  brill- 
iant than  if  the  earth  were  moving  from  c  to  d,  as 
we  should  then  be  much  nearer  it  during  its  greatest 
illumination. 

Number  of  Comets. — Kepler  remarks  that  "there 
are  as  many  comets  in  the  heavens  as  fish  in  the 
sea."  Arago,  basing  his  calculations  on  the  number 
known  to  exist  between  the  sun  and  Mercury,  has 
estimated  that  there  are  17,500,000  within  the  solar 
system.  Of  this  vast  number,  few  are  visible  to  the 
naked  eye,  and  a  still  less  number  attract  observa- 
tion, owing  to  their  inferior  size  and  brilliancy. 
Many  are  doubtless  lost  to  our  sight  by  being  above 
the  horizon  in  the  daytime.  During  the  eclipse  of 
1882,  Lockyer,  who  was  in  Egypt  to  take  observa- 
tions, saw  a  brilliant  comet  near  the  sun. 

Orbits  of  the  Comets. — Comets  form  a  part  of  the 
solar  system,  and  are  subject  to  the  laws  of  gravita- 
tion. Like  the  planets,  they  revolve  around  the  sun, 
though  they  differ  in  the  form  of  their  orbits.  While 
the  planets  move  in  paths  varying  but  little  from 
circular,  and  thus  never  depart  so  far  from  the  sun  as 
to  be  invisible  to  us,  the  comets  travel  in  extremely 
elongated  (flattened)  ellipses,  so  that  they  can  be  ob- 
served by  us  through  only  a  small  portion  of  their 
paths. 

In  Fig.  79  are  represented  the  three  general  classes 
of  cometary  orbits.  A  comet  traveling  along  an 
elliptical  orbit,  though  it  may  pass  far  from  the  sun, 
will  yet  return  within  a  fixed  time ;  one  pursuing 
either  a  parabolic  or  hyperbolic  curve  cannot  return, 
as  the  two  sides  separate  from  each  other  more  and 


COMETS. 


169 


more.  Many  of  the  comets  of  the  first  class  have  been 
calculated,  and  they  have  repeatedly  visited  our  por- 
tion of  the  heavens  ;  while  those  of  the  other  classes, 
having  once  visited  our  system,  go  away  forever, 


Fig.  79. 


Three  Forms  of  Cometary  Orbits. 

seeking  perhaps  in  the  far-off  space  another  sun, 
which  in  turn  they  will  abandon  as  they  have  our  own. 
Calculation  of  a  Comet's  Return. — As  we  can  c"" 
serve  so  small  a  proportion  of  the  entire  orbit,  ^^  p^^- 
very  difficult,  indeed  oftentimes  impossible,  to  d* 


idO 


THE  SOLAR  SYStEiM. 


whether  it  is  an  hyperbola,  an  ellipse,  or  a  parabola. 
A  few  are  known  to  move  in  elliptical  paths,  and 
their  orbits  have  been  so  accurately  computed  that  it 
is  possible  to  predict  the  time  of  their  appearance. 
The  other  comets  may  never  return,  or  at  least  not 

Fig.  80. 


Projections  of  a  few  Cometary  Orbits  on  the  Plane  of  the  Ecliptic, 

for  centuries  hence.  They  may  be  paying  our  sun 
their  first  visit ;  or,  if  they  have  swept  through  the 
"^vlar  system  before,  it  may  have  been  at  so  remote  a 
eitie  that  no  record  is  preserved,  even  if  it  were  not 
as  tre  the  creation  of  man.     Under  these  circum- 


COMEfS.  131 

stances,  it  is  difficult  to  determine  the  place  of  these 
apparently  erratic  wanderers ;  yet,  in  spite  of  all 
these  obstacles,  some  have  been  tracked  into  space 
far  beyond  the  telescopic  view.  For  example,  the 
comet  of  1844  is  announced  to  pay  a  visit  to  the 
astronomers  of  the  year  of  our  Lord  101,844.  The 
period  of  the  comet  of  1744,  is  fixed  at  123,683 
years. 

Distance  from  the  Sun. — Some  comets  at  their  peri- 
helion sweep  near  the  sun.  Thus  the  one  of  1680  came 
where  the  temperature  was  estimated  by  Newton  to 
be  about  2,000  times  that  of  red-hot  iron.  *  The  near- 
est approach  known  is  that  of  the  comet  of  1843, 
whose  perihelion  distance  was  but  about  30,000  miles 
from  the  surface  of  the  sun  ;  in  fact,  it  doubled 
around  that  body  in  two-hours  time.  (Guillemin.)f 
The  greatest  aphelion  distance  yet  estimated  is  that 
of  the  comet  of  1844,  which  is  over  400,000,000,000 
miles.  The  velocity  varies,  of  course,  with  the 
position  in  the  orbit.  The  comet  of  1680  moved  in 
perihelion  at  the  rate  of  over  two  hundred  and 
seventy-seven  miles  per  second  ;  while  in  aphelion 
its  velocity  is  only  about  six  miles  per  hour. 

Density  of  Comets. — The  quantity  of  matter  con- 
tained in  a  comet  is  exceedingly  small.     Even  tele 
scopic  stars  are  visible  through  the  densest  part.    The 
comet  of   1770  became  entangled  among  Jupiter's 

*  The  comet  of  1680  excited  such  terror  in  Europe  that  a  medal  was  struck,  to  quiet 
the  fears  of  the  people.  The  inscription  read  thus:  "The  star  threatens  evil  things; 
trust  only  !  God  will  turn  them  to  good."  Newton  calculated  the  oi  bit  of  this  comet 
and  proved  that  the  comet  moves  around  the  sun  in  obedience  to  the  law  of  gravity. 

t  The  comet  of  1843  excited  much  interest  in  this  country  since  one  Miller  had  pre- 
dicted that  the  end  of  the  world  would  come  in  that  year;  his  followers  imagined  this 
comet  presaged  the  destruction  of  all  things. 


192  THE   SOLAR  SYSTEM. 

moons,  and  remained  there  four  months  without  in 
terf eriug  with  their  movements  ;  indeed,  so  far  from 
that,  its  own  orbit  was  so  much  changed  by  their 
proximity,  that,  from  a  periodical  return  of  5f  years, 
it  has  not  been  seen  since.  We  have  good  reason  to 
suppose  that  the  earth,  in  1861,  passed  through  the 
tail  of  a  comet,  its  presence  being  indicated  only  by 
a  peculiar  phosphorescent  mist.  So  that  even  should 
our  earth  run  full-tilt  against  a  comet,  the  shock 
might  be  quite  imperceptible.*  Still,  however 
lightly  we  may  speak  of  the  probability  of  such  a 
collision,  we  must  remember  that  there  are  comets 
of  greater  solidity.  Donati's,  for  instance,  is  esti- 
mated by  some  to  be  about  yws  the  mass  of  the  earth. 
The  concussion  of  such  a  body,  moving  with  the 
speed  of  a  cannon-ball,  would  undoubtedly  produce 
a  very  sensible  effect. 

It  is  not  determined  whether  comets  shine  by  their 
own  or  by  reflected  light.  If,  however,  their  nuclei 
consist  of  white-hot  matter,  a  passage  through  such 
a  furnace  would  be  anything  but  desirable  or  satis- 
factory. After  all  the  calculations  of  Astronomy, 
our  only  safety  lies  in  that  Almighty  Power  which 
traces  the  path  and  guides  the  course  alike  of 
planets  and  comets  :  He,  whose  eye  marks  the  fall 

*  "  However  dangerous  might  be  the  shock  of  a  comet,  it  might  be  so  slight  that  it 
would  onlr  do  damage  to  that  part  of  the  earth  where  it  actuaUy  struck ;  perhaps, 
even,  we  might  cry  quits,  if,  wliile  one  kingdom  were  devastated,  the  rest  of  the  earth 
were  to  enjoy  the  rarities  which  a  body  coming  from  so  far  might  bring  to  it.  Perhaps 
we  should  be  very  surprised  to  find  that  the  debrU  of  these  masses  that  we  despised 
were  formed  of  gold  or  diamonds  ;  but  who  would  be  the  more  astonished — we  or  the 
comet-dwellers  who  would  be  cast  upon  our  earth?  What  strange  beings  each  would 
find  the  other?"    Lettre  sur  la  Comete,  par  M.  De  Manpertuis. 

Young  says,  "  It  seems,  on  the  whole,  more  probable  that  a  comet  is  only  a  cloud  of 
dust  and  vapor-  a  smcike-wreath — than  that  there  is  at  the  center  any  solid  kernel.  A 
comet  is  a  mere  airy  nothing." 


COMETS.  193 

of  the  sparrow,  sees  as  well  the  flight  of*  the  worlds 
He  has  created. 

Variations  in  Form  and  Dimensions. — Comets  ap- 
pear to  be  subject  to  constant  variations.  They  are 
now  thought  generally  to  decrease  in  brilliancy  at 
each  successive  revolution  about  the  sun.  The  same 
comet  may  present  itself  sometimes  with  a  tail,  and 
sometimes  without.  When  the  comet  first  appears, 
there  is  commonly  no  tail  visible,  and  the  light  is 
faint.  As  it  approaches  the  sun,  however,  its  bright- 
ness increases,  the  tail  shoots  out  from  the  coma, 
and  grows  daily  in  length  and  splendor.  Supernu- 
merary tails,  shorter  and  less  distinct  than  the  prin- 
cipal one,  dart  out,  but  they  generally  soon  disap- 
pear, as  if  from  lack  of  material.  The  tail  of  the 
comet  of  1843,  just  after  the  perihelion,  increased  in 
length  5,000,000  miles  per  day.  As  the  tail  thus  ex- 
tended, the  nucleus  was  correspondingly  contracted, 
so  that  this  comet  actually  "  exhausted  its  head  in 
the  manufacture  of  its  own  tail." 

Remarkable  Comets. — Among  the  many  comets 
celebrated  in  history,  we  shall  notice  only  some  of 
those  that  have  appeared  in  the  present  century. 
The  great  comet  of  1811  was  a  magnificent  spectacle.  * 
The  head  was  112,000  miles  in  diameter  ;  the  nucleus 
was  400  miles ;  while  the  tail,  of  a  beautiful  fan- 
shape,  stretched  out  112,000,000 miles.  "The aphelion 
distance  of  this  comet  is  fourteen  times  that  of  Nep- 
tune, or  40,000,000,000  miles.  It  is  announced  to  re- 
turn in  thirty  centuries  ! "    To  what  profound  depths 

*  This  was  considered  by  tlie  Russians  to  presage  Napoleon's  Invasion. 


194  THE  SOLAR  SYSTEM. 

of  space,  beyond  the  solar  system,  beyond  the  reach 
of  the  telescope,  must  such  a  journey  extend  ! 

Fig.  81. 


Coggia's  Comet,  1S7L 

The  Comet  of  1835  is  known  as  Halley's  comet. 
This  is  remarkable  as  being  the  first  comet  whose 
period  of  revolution  was  satisfactorily  established. 
Dr.  Halley,  on  examining  the  accounts  of  the  great 
comets  of  1531,  1607,  and  1682,  suspected  that  they 
were  the  reappearances  of  the  same  comet,  whose 
period  he  fixed  at  about  75  years.*    He  finally  ven- 

•  The  history  of  this  comet,  as  it  has  been  traced  back  by  it^  period  of  seventy-five 
years,  is  quite  eventful.  It  was  seen  in  England  in  1066,  when  it  was  looked  upon  with 
dread  as  the  forerunner  of  the  \ictorj'  of  William  of  Normandy.  It  was  then  equal  to 
the  full  moon  in  size.     In  1456,  its  tail  reached  from  the  horizon  to  the  zenith.    It  was 


COMETS.  195 

tured  to  predict  the  return  of  the  coined  at  near  the 
end  of  1758  or  beginning  of  1759.  Although  Halley 
did  not  live  to  see  his  prophecy  fulfilled,  great  in- 
terest was  felt  in  the  result.  It  was  not  destined, 
however,  for  a  professional  astronomer  to  be  the 

Fig.  8S. 


Donati's  Coniel. 

first  to  detect  the  comet.  A  peasant  near  Dresden 
saw  it  on  Christmas  night,  1758. 

supposed  to  indicate  the  success  of  Mahomet  II.,  who  had  already  taken  Constantinople, 
and  then  threatened  the  whole  Christian  world.  Pope  Calixtus  III.,  therefore,  ordered 
extra  Ave  Marias  to  be  repeated  by  everybody,  and  also  the  church  bells  to  be  rung 
daily  at  noon  (whence  originated  the  custom  now  so  universal).  A  prayer  was  added  as 
follows  :  "Lord,  save  us  from  the  devil,  the  Turk,  and  the  comet."  In  1223,  it  was  con- 
sidered the  precursor  of  the  death  of  Philip  Augustus  of  France.  The  first  recorded  ap- 
pearance of  Halley's  comet  was  b.  c.  130,  when  it  was  supposed  to  herald  the  birth  of 
Mithridates. 


19G  THE   SOLAR  SYSTEM. 

The  Comet  of  1843  was  so  brilliant  that  it  was 
visible  in  full  daylight.  It  was  so  near  the  sun  at 
perihelion  as  "  almost  to  graze  his  surface." 

Encke's  Comet  has  a  period  of  only  3^  years.  A 
most  interesting  discovery  has  been  made  from  ob- 
servations upon  its  motion.  The  comet  returns  each 
time  to  its  perihelion  about  3^  hours  earlier  than  the 
calculations  indicate.  Hence,  Prof.  Encke  has  been 
led  to  conjecture  chat  space  is  filled  with  a  thin, 
ethereal  medium  capable  of  diminishing  the  centri- 
fugal force,  and  thus  contracting  the  orbit  of  a  comet. 

DoNATi's  Comet  (1858)  was  the  subject  of  universal 
wonder.  When  first  discovered,  in  June,  it  was 
240,000,000  miles  from  the  earth.  In  August,  traces 
of  a  tail  were  noticed,  which  expanded  in  October  to 
about  50,000,000  miles  in  length.  This  comet,  though 
small,  has  never  been  exceeded  in  the  brilliancy  of 
the  nucleus  and  the  graceful  curvature  of  the  tail. 
It  will  return  in  about  2,000  years. 

The  "  Great  Comet  of  1882  "  had,  soon  after  pass- 
ing its  perihelion,  a  nucleus  as  bright  as  a  star  of  the 
1st  magnitude,  and  a  tail  60,000,000  miles  long.  The 
aphelion  of  its  orbit  is  six  times  further  than  Nep- 
tune from  the  sun,  and  the  comet's  period  is  esti- 
mated at  between  eight  and  nine  centuries. 


V.    ZODIACAL    LIGHT. 

Description. — If  we  watch  the  western  horizon  in 
March  or  April,  just  after  sunset,  we  shall  sometimes 
see  the  short  twilight  of  that  season  illuminated  by 


ZODIACAL  LIGHT. 


197 


a  faint  nebulous  light,  of  a  conical  shape,  flashing 
upward,  often  as  high  as  the  Pleiades.  In  Septem- 
ber and  October,  at  early  dawn,  the  same  appearance 


Fig.  SS. 


Zodiacal  Light. 

can  be  detected  near  the  eastern  horizon.  The  light 
can  be  seen  in  this  latitude  only  on  the  most  favor- 
able evenings,  when  the  sky  is  clear  and  the  moon 
absent.  Even  then,  it  will  be  frequently  confounded 
with  the  Milky  Way  or  auroral  lights.     At  the  base. 


198  THE  SOLAR  SYSTEM. 

it  is  of  a  reddish  hue,  where  it  is  so  bright  as  often 
to  efface  the  sraaller  stars.  In  tropical  regions,  the 
zodiacal  light  is  perpetual,  and  shines  with  a  bril- 
Uancy  sufficient,  says  Humboldt,  to  cast  a  sensible 
glow  on  the  opposite  part  of  the  heavens. 

Origin. — The  commonly-received  opinion  is,  that  it 
is  caused  by  a  faint,  cloud-like  ring,  perhaps  a 
meteoric  zone,  that  surrounds  the  sun,  and  becomes 
visible  to  us  only  when  the  sun  himself  is  hidden 
below  the  horizon.  Others  maintain  that,  since  it 
has  been  seen  in  tropical  regions  in  the  east  and  the 
west  simultaneously,  it  can  be  explained  only  on  the 
theory  of  a  "  nebulous  ring  that  surrounds  the  earth 
within  the  orbit  of  the  moon." 


PRACTICAL    QUESTIONS. 

1.  Would  the  earth  rise  and  set  to  a  Lunarian  ? 

2.  Could  there  be  a  transit  of  Neptune  ? 

3.  Why  does  Mars's  inner-moon  rise  in  the  Avest  ? 

4.  In  what  part  of  the  sky  do  you  always  look  for  the  planets  ? 

5.  Show  how  it  was  impossible  for  the  darkness  that  occurred  at  the 
time  of  the  Cmcifixion  of  Christ  to  have  been  caused  by  an  eclipse  of  the 
sun. 

6.  Is  there  any  danger  of  a  collision  between  the  earth  and  a  comet  ? 

7.  How  are  aerolites  distinguished  ? 

8.  "When  do  we  see  the  old  moon  in  the  west  after  sunrise  ? 

9.  When  do  we  see  the  moon  high  in  the  eastern  sky  in  the  afternoon 
before  the  sun  sets  ? 

10.  When  is  a  planet  morning,  and  when  evening,  star  ? 

11.  Is  the  sun  really  hotter  in  summer  than  in  winter? 

12.  Why  is  a  planet  invisible  at  conjunction  ? 

13.  Must  an  inferior  planet  always  be  in  the  same  part  of  the  sky  as  tb? 
sun  ?  A  superior  planet  J 


PRACTICAL  QUESTIONS.  199 

14.  Why,  in  summer,  does  the  sun,  at  rising  and  at  setting,  shine  on 
the  north  side  of  certain  houses  ? 

15.  What  effect  does  the  volume  of  a  planet  have  upon  the  force  of 
gravity  at  its  surface  ? 

16.  In  what  part  of  the  heavens  do  we  see  the  new  moon  ?  The  old 
inoon  ?     The  crescent  moon  ? 

17.  What  is  the  Golden  Number  in  the  almanac  ? 

18.  Why  do  we  have  more  lunar  than  solar  eclipses  ? 

19.  In  what  direction  do  the  horns  of  the  moon  turn  ? 

20.  Is  the  "  tidal- wave  "  an  actual  movement  of  the  water  ? 

21.  Why  does  the  sun  "  cross  the  line  "  in  some  years  on  March  21,  and, 
in  others,  on  March  22  ? 

22.  Do  we  ever  see  the  sun  where  it  really  is  ? 

23.  At  Edinburgh,  Scotland,  there  are  times  when  the  sun  rises  at  3^ 
o'clock  A.  M.  and  sets  at  8^  o'clock  p.  M.,  and  the  twilight  lasts  the  entire 
night.     When  and  why  is  this  ? 

24.  Which  is  the  longest  day  of  the  year  ? 

25.  Is  the  moon  nearer  to  us  when  it  is  at  the  horizon,  or  at  the  zenith  ? 

26.  How  many  solar  eclipses  would  happen  each  year  if  the  orbits  of  the 
sun  and  the  moon  were  in  the  same  plane  ? 

27.  Is  there  any  heat  in  moonlight  ? 

28.  Can  we  see  the  moon  during  a  total  eclipse  ? 

29.  Which  of  the  planets  are  repeating  a  portion  of  the  earth's  history  ? 

30.  How  many  times  does  the  moon  turn  on  its  axis  each  year  ? 

31.  Can  you  explain  the  different  signs  used  in  the  almanac  ? 

32.  Show  how  the  moon  is  a  prophecy  of  the  earth's  future. 

33.  Does  the  sun  really  rise  and  set  ? 

34.  Are  the  bright  portions  of  the  moon  mountains  or  plains  ? 

35.  Which  of  the  heavenly  bodies  are  self-luminous  ? 

36.  VVliy  is  not  a  solar  eclipse  visible  on  the  whole  earth  ? 

37.  What  is  meant  by  the  "  mean  distance  "  of  a  planet  ? 

38.  What  keeps  the  earth  in  motion  around  the  sun  ? 

39.  Do  we  ever  see  the  sun  after  it  sets  ? 

40.  When  does  the  earth  move  the  most  rapidly  in  its  orbit  ? 

41.  Have  we  conclusive  evidence  that  any  planet  is  inhabited  ? 

42.  When  is  the  twilight  the  longest?     The  shortest  ?     Whj^  ? 

43.  What  is  a  moon  "j 


200  PRACTICAL  QUESTIONS. 

44.  To  a  person  in  the  south  temperate  zone,  wliere  would  the  sun  be 
at  noon  ? 

45.  Is  it  correct  to  say  that  the  moon  revolves  about  the  earth,  when  we 
know  that,  according  to  the  law  of  Physics,  they  must  both  revolve  about 
their  common  center  of  gravity  ?* 

46.  During  a  transit  of  Venus,  do  we  see  the  body  of  the  planet  itself  on 
the  face  of  the  sun  ? 

47.  How  many  real  motions  has  the  sun  ?     How  many  apparent  ones  ? 

48.  How  many  real  motions  has  the  earth  ? 

49.  Can  an  inferior  planet  have  an  elongation  of  90°  ? 

50.  How  do  we  know  the  intensity  of  the  sun's  light  on  the  surface  of 
any  of  the  planets  ? 

51.  Why  is  the  Tropic  of  Cancer  placed  where  it  is  ? 

52.  What  planets  would  float  in  water  ? 

53.  How  must  the  moons  of  Jupiter  appear  during  their  transit  across 
the  disk  of  that  planet  ? 

54.  "The  shadow  of  the  satellite  precedes  the  satellite  itself  when  Ju- 
piter is  passing  from  conjunction  to  oi)position,  but  follows  it  between 
opposition  and  conjunction."     Explain. 

55.  What  facts  point  to  the  conclusion  that  Mars  may,  perhaps,  have 
passed  his  planetary  prime  ? 

56.  Why  may  we  conceive  that  Saturn  and  Jupiter  are  yet  in  their 
planetary  youth  ? 

57.  Show  how,  if  the  Nebular  Hypothesis  (p.  256)  be  accepted,  the 
fashioning  of  a  planet  must  require  an  enormous  length  of  time. 

58.  Do  we  know  the  cause  of  gravitation  ? 

*  "Strictly  speaking,  the  moon  does  not  revolve  around  the  earth,  any  more  than 
the  earth  around  the  moon  ;  but,  by  the  principle  of  action  and  reaction,  the  center  of 
each  body  moves  around  the  common  center  of  gravity  of  the  two  bodies.  The  eartli 
being  eighty  tiniis  as  heavy  as  the  moon,  this  center  is  situated  within  the  former, 
about  three-quarters  of  the  way  from  its  center  to  its  surface."— i^ewcomh's  Astronomy, 
p.  91. 


III. 
THE    SIDEREAL   SYSTEM. 


•'  He  telleth  the  number  of  the  stars  /  He  calleth  them  all  by  theit 
names," 

Psalm  cxlvii.  4. 


L  The  Stabs. 


f  1.  fixED  Staks  KOt  SEEir. 

2.  Parallax  ksi>  Distawce. 

3.  Motion. 

4.  Stars  ake  Srss. 

5.  OcB  Si-s  A  Star. 

6.  Solar  System  ix  Monos. 

7.  Number  of  Stars. 

8.  Scintillation. 

9.  Magnitude. 

10.  Cause  of  Difference  in  Brightness. 

11.  Sames. 

VI.  The  Constellations. 

13.  Invention  of  Constellations. 

14.  Signs  and  Constellations  not  agreeing 

15.  Perma>"ence  of  Constellations. 
Ifi.  Y.^lit;  of  Stars. 


17.  Ancient  Views. 
IS.  Three  Zones. 


IL  The  Constel- 
lations..... 


(   1.  How  traced. 

•2.  Ursa  Major. 

3.  Ursa  Minor. 

1.  Northern  Cir-  ) 

; 

CCMPOLAR  Con-  ' 

STELLATIONS.for 

4.  Draco.                   > 

Latitude  of  New 

5.  Cepheus. 

York. I   <J.  Cassiopeia. 

1.    How  TRACED. 

2.  Perseu:^. 

3.    AiJDROMEDA. 

4.  Aries. 

5.  Taubcs. 

6.  Auriga. 

7.  Pisces. 

8.  Cetus. 

9.  Gemini. 

10.  Orion. 

11.  Canis. 

12.  Leo. 

2.    EtJCATORIAL 

13.  Cancer. 

Constellations. 

14.  YiRoo. 

15.  Hydra. 

16.  Canes  Venatcci. 

17.  Berenice's  Hair, 

IS.  Bootes. 

19.  Hercitles. 

20.  Corona. 

21.  Serpentarius. 

22.  Libra. 

23.  SAonT.uiius. 

24.  Capricornvs. 

3.  The   SoiTHERN 

25.  Cygnus. 

,     Con-stell-\tion-s. 

l,2t<.  Lyra. 

^  a.  Description. 
!   6.  Principal  Stare. 
V  c.  Mythological  Hist- 
'  d.  Distance  of  Polaris. 

)  t.  Latitude. 


a.  Description. 
6.  Principal  Stare, 
c.  M>-thological  Hist 


in.  Double  St.uis,  St.vr  Clusters, 
Colored  Stars,  etc  


IV.  Celestial  Chemistry 


VL  Celestial  MEASUBEMt-vre . 


r  1-^.  Double  Stars,  Colored  Stars,  Variabli 
I  Stars,  Temporary  Stars,  Star  Clu-s 

J  TERS,  NEBUL.t:. 

\  7.  Magellanic  Clouds. 

I  S.  The  Milky  Way. 

V9.  The  Nebular  Hypothesis, 

1.  Spectrum  Analysis. 

2.  Spectroscope. 

3.  Revelations  Concerning  Srs. 

4.  Concerning  Stars. 

1  5.  Concerning  Nebul.e. 
V^6.  Concerning  Solar  Fl.oie3. 
/  1.  Sidereal. 
J  2.  Solar. 
1  3.  Mean  Solar. 
I  4.  Sun-dial,  etc. 

'  1    To  Find  Distance  of  Pl-Ocets  from  sun. 
I  •'!  To  Fint)  Moons  Distance  from  Earth. 
^  3.  To  Find  Sun's  Distance  from  Earth. 
L  4.  To  FiifD  LoKcrruDK  op  a  Place,  etc. 


THE  SIDEREAL  SYSTEM. 


I.     THE   STARS. 


IN  our  celestial  journey  we  have  reached  Nep- 
tune, the  sentinel  outpost  of  the  solar  system. 
We  are  now  nearly  2,800,000,000  miles  from  our 
sun.  Yet  we  are  apparently  no  nearer  the  fixed  stars 
than  when  we  started.  They  twinkle  as  serenely 
there  in  the  far-off  sky  as  to  us  here  on  the  earth. 
The  heavens  by  night,  with  the  exception  of  a  few 
changes  in  the  planets,  look  familiar.  Between  them 
and  us  there  is  still  a  vast  chasm  which  no  imagi- 
nation can  bridge ;  a  distance  so  immense  that  figures 
are  meaningless,  and  we  can  only  call  it  space, — so 
profound  that  to  us  it  is  limitless,  though  beyond  we 
see  other  worlds  twinkling,  like  distant  lights  over  a 
waste  of  waters. 

We  never  see  the  Stars. — This  assertion  seems 
paradoxical,  yet  it  is  strictly  true.  So  far  are  the 
stars  removed  from  us,  that  we  see  only  the  light  they 
send,  but  not  the  surface  of  the  worlds  themselves. 
They  are  merely  glittering  points  of  light.  The  most 
powerful  telescope  fails  to  produce  a  sensible  disk. 
This  constitutes  a  marked  difference  between  a  planet 
and  a  fixed  star. 


204  THE   SIDEREAL   SYSTEM. 

The  Annual  Parallax  of  tne  Fixed  Stars. — When 
speaking  of  this  subject  on  page  121,  we  said  that 
186,000,000  miles,  or  the  diameter  of  the  earth's  orbit, 
is  the  unit  for  measuring  the  parallax  of  the  fixed 
stars.  Yet  when  the  stars  are  viewed  from  even 
these  extreme  points,  they  manifest  so  slight  a 
change  of  place,  that  to  estimate  it  is  one  of  the  most 
delicate  feats  of  astronomy. 

At  the  present  time,  it  is  considered  that  the  star 
Alpha  (a)  Centauri  in  the  southern  heavens  is  the 
nearest  to  the  earth.  Its  parallax  is  judged  to  be 
about  1".  Its  distance  is  more  than  200,000  times  that 
of  the  earth  from  the  sun,  or  twenty  trillions  of  miles. 
This  is  probably  by  no  means  its  actual  distance, 
but  merely  the  limit  within  which  it  cannot  be,  but 
beyond  which  it  must  be.* 

These  figures  convey  to  our  mind  no  idea  of  dis- 
tance. Our  imagination  fails  to  grasp  the  thought, 
or  to  picture  the  vast  void  across  which  we  are  gaz- 
ing. We  remember  that  light  moves  at  the  rate  of 
186,000  miles  per  second.  A  ray  at  that  speed  would, 
in  one  day,  plunge  out  into  the  abyss  beyond  Nep- 
tune six  times  the  distance  of  that  planet  from  the 
sun.  Yet  it  must  sweep  on  at  this  prodigious  speed, 
day  and  night,  for  over  3^  years  to  span  the  gulf 

*  David  Gill,  the  Royal  Astronomer  at  the  Cape  of  Good  Hope,  has  recently  deter- 
mined the  parallax  of  a  Centauri  to  be  0".75.  This  would  make  its  distance  275,0CC 
astronomical  units.  275,000  x  93,000,000  miles  =  over  25^  trillion  miles.  Light 
would  require  about  i\  years  to  travel  this  enormous  distance.  Vega's  parallax  is 
placed  at  not  far  from  0".2,  which  indicates  a  distance  of  about  1,000,000  astronomical 
units.  Hence,  Vega  shines  upon  us  from  the  inconceivable  distance  of  niiiety-three 
trillion  mile.i.'  The  parallax  of  Sirius  has  been  variously  estimated  at  from  0".16  to 
0".38.  Newcomb  places  this  star  at  more  than  a  million  radii  of  the  earth's  orbit  away 
from  us,  yet  its  light  is  four  times  as  brilliant  as  that  of  any  other  star.  The  difficulty 
of  measuring  the  stellar  parallax  may  be  judged  from  the  fact  that  1"  measures  the  angle 
at  wliich  a  globe  three-tenths  of  an  inch  in  diameter  would  be  seen  when  a  mile  away. 


.    THE  STARS.  205 

and  reach  a  stopping  point  at  the  nearest  fixed  star. 
It  has  been  estimated  that  the  average  time  re- 
quired for  the  light  of  the  smallest  stars  which  are 
visible  to  the  naked  eye  to  reach  the  earth  is  about 
125  years.  What,  then,  shall  we  say  of  those  far- 
distant  ones,  whose  faint  light  appears  as  a  mere 
fleecy  whiteness  even  in  the  most  powerful  tele- 
scopes? The  conclusion  is  irresistible,  that  the  light 
we  receive  set  out  on  its  sidereal  journey  far  back  in 
the  past,  perhaps  before  the  creation  of  man! 

Motion  of  the  Fixed  Stars. — It  will  aid  us  still 
further  in  comprehending  the  immense  distances  of 
the  stars,  to  learn  that,  though  they  seem  to  be  fixed, 
they  are  moving  much  more  swiftly  than  any  of 
the  planets.  Thus,  Arcturus  flies  through  space  at 
the  astonishing  rate  of  200,000  miles  per  hour,  or 
nearly  twice  that  of  Mercury,  and  more  than  three 
times  that  of  the  earth.  Yet,  through  all  our  life- 
time, we  shall  never  be  able  to  detect  any  change  in 
its  position.  "  It  requires  three  centuries  for  it  to 
move  over  the  starry  vault  a  space  equal  to  the 
moon's  apparent  diameter." 

The  Stars  are  Suns.— The  vast  distance  at  which 
the  stars  are  known  to  be,  precludes  the  thought  of 
their  shining,  like  the  planets  or  the  moon,  by  reflect- 
ing back  the  light  of  our  sun.  They  must  be  self- 
luminous,  and  are  doubtless  each  the  center  of  a 
system  of  planets  and  satellites. 

Our  Sun  a  Star. — As  we  see  only  the  suns  of  these 
distant  systems,  so  their  inhabitants  see  only  the  sun 
of  our  system,  and  that  as  a  small  star. 

Our  System  in  Motion. — Like  all  the  other  stars. 


m 


THE  SIDEREAL  SYSTEM. 


our  sun  is  in  motion.  It  is  sweeping  onward,  with 
its  retinue  of  worlds,  150,000,000  miles  per  year,  toward 
a  point  in  the  constellation  Hercules.  The  Pleiades 
has  been  thought  to  be  the  center  around  which  this 


Mg.  8L 


A  part  of  the  Constellation  of  the  Twins. 


great  movement  is  taking  place,  but  most  astron- 
omers consider  the  idea  as  a  mere  speculation. 

The  Number  of  the  Fixed  Stars.— When  we  look  at 
the  heavens  on  a  clear  night,  the  stars  seem  innumer- 


tH£  STAflg.  ^0? 

able.  To  count  them,  one  would  think  almost  as  in- 
terminable a  task  as  to  number  the  leaves  on  the 
trees.  It  is,  therefore,  somewhat  startling  to  learn 
that  the  entire  number  visible  to  the  most  piercing 
eyesight  does  not  exceed  6,000,  while  few  can  dis- 
cern more  than  4,000.*  The  number,  however,  which 
may  be  seen  with  a  telescope  is  marvellous.  In  Fig. 
84,  is  shown  a  portion  of  the  heavens  where  the 
naked  eye  sees  but  six  stars.  Could  we  examine  the 
same  region  of  the  sky  with  more  powerful  instru- 
ments, new  constellations  would  doubtless  be  des- 
cried in  the  infinite  depths  of  space. 

Scintillation. — The  twinkling  of  the  fixed  stars  is 
due  to  what  is  termed  in  Physics  the  "Interference 
of  Light."  The  air,  being  unequally  dense,  warm, 
and  moist  in  its  various  strata,  transmits  very  irregu- 
larly the  different  colors  of  which  white  light  is  com- 
posed. Now  one  color  prevails  over  the  rest,  and 
now  another,  so  that  the  star  appears  to  alter  its  hue 
incessantly.  As  the  purity  and  density  of  the  air 
vary,  the  twinkling  of  the  stars  also  changes,  and, 
therefore,  it  is  always  greatest  near  the  horizon,  t 

Magnitude  of  the  Stars. — As  the  telescope  reveals 
no  disk  of  even  the  nearest  stars,  we  know  nothing 
of  their  comparative  size.  The  finest  spider's  thread, 
placed  at  the  focus  of  the  instrument,  hides  the  star 
from  the  eye.     When  the  moon  passes  in  front  of  a 

*  This  illusion  may  be  easily  exiilained,  when  we  remember  how  the  impression  of  a 
bright  light  remains  upon  the  retina,  as  in  the  whirling  of  a  firebrand. 

t  Humboldt  says  that  at  Cumana,  in  South  America,  where  the  air  is  remarkably 
pure  and  uniform  in  density,  the  stars  cease  to  twinkle  after  tliey  have  risen  15°  above 
the  horizon.  This  gives  to  the  celestial  vault  a  peculiarly  calm  and  soft  appearance. — It 
should  he  noticed  that  interference  occurs  only  when  the  light  emanates  from  a  point. 
A  body  that  subtends  a  visual  angle,  i.  e.,  has  a  sensible  disk,  like  a  planet,  cannot  twinkle. 


208 


THE  SIDEREAL  SYSTEM. 


star,  the  occultation  is  instantaneous,  and  not  gradual, 
as  in  the  case  of  the  planets.  Classification  depends, 
therefore,  merely  upon  their  relative  brightness. 
The  most  conspicuous  are  termed  stars  of  the  first 


Fin.  sr,. 


magnitude;  of  these  there  are  about  twenty.  The 
number  of  second-magnitude  stars  in  the  entire 
heavens  is  sixty -five  ;  of  the  third,  about  200 ;  of  the 
fifth,  1,100 ;  of  the  sixth,  3,200 ;  of  the  seventh,  13,000 ; 
of  the  eighth,  40,000  ;  and  of  the  ninth,  142,000.  Few 
persons  can  see  smaller  stars  than  those  of  the  fifth 
or  sixth  magnitude. 

The  DiflFerence  in  the  Brightness  of  the  stars  may 
result  from  a  dilBference  in  their  distance,  size,  or 
intrinsic  brightness.  Hence  it  follows  that  the  faint- 
est stars  may  not  be  the  most  distant  from  the  earth. 

Names  of  Stars. — Many  of  the  brightest  stars 
received  proper  names  at  an  early  date ;  as  Sirius, 
Arcturus.  The  chief  stars  of  each  constellation  are 
distinguished  by  the  letters  of  the  Greek  alphabet ; 

The  Greek  Alphabet. 


A 

a 

Alpha 

I 

I 

Iota 

P    p     Rlio 

B 

/3 

Beta 

K 

K 

Kappa 

2    <T    Sigma 

r 

V 

Gamma 

A 

A 

Lambda 

T    T    Tau 

A 

8 

Delta 

M 

^ 

Mu 

Y   V    Upsilon 

E 

e 

Epsilon 

N 

V 

Nu 

*   <t>    Phi 

Z 

i 

Zeta 

E 

f 

Xi 

X  X    Chi 

H 

V 

Eta 

O 

0 

Omicron 

♦  ^    Psi 

9 

0 

Theto 

n 

ir 

Pi 

Q  0}   Omega 

THE  STAKS.  209 

the  brightest  one  being  usually  called  Alpha  (a),  the 
next  Beta  (j"^),  etc., — the  name  of  the  constellation,  in 
the  genitive  case,  being  put  after  each.  Ex.,  a  Arie- 
tis,  (3  Lyrse.* 

Star  catalogues  are  issued,  containing  the  stars 
arranged  in  the  order  of  their  Right  Ascension,  and 
numbered  for  convenience  of  reference.  Argelan- 
der's  Charts  have  334,188  stars  marked  in  the  north- 
ern hemisphere. 

The  Constellations. — From  the  earliest  ages,  the 
stars  have  been  arranged  in  constellations,  for  the 
purpose  of  more  readily  distinguishing  them.  Some 
of  these  groups  were  named  from  their  supposed  re- 
semblance to  certain  figures,  such  as  perching  birds, 
pugnacious  bulls,  or  contorted  snakes,  while  others 
do  honor  to  the  memory  of  classic  heroes. 

"Thus  monstrous  forms,  o'er  heaven's  nocturnal  arch, 
Seen  by  the  sage,  in  pomp  celestial  march  ; 
See  Aries  there  his  glittering  bow  unfold, 
And  raging  Taurus  toss  liis  horns  of  gold  ; 
With  bended  bow  the  sullen  Archer  lowers, 
And  there  Aquarius  comes  with  all  his  showers  ; 
Lions  and  Centaurs,  Gorgons,  Hj'dras  rise. 
And  gods  and  lieroes  blaze  along  the  skies." 

With  a  few  exceptions,  the  likeness  is  purely  fan- 
ciful. Not  only  are  the  figures  uncouth,  and  the 
origin  often  frivolous,  but  the  boundaries  are  not 
distinct.  Stars  occur  under  different  names  ;  while 
one  constellation  encroaches  upon  another,  f  Though, 

*  This  means  a  of  Aries,  /3  or  Lyra  ;  the  genitive  case  in  Latin  being  equivalent  to  the 
preposition  of. 

t  Chambers  well  remarks,  "Aries  should  not  have  a  horn  in  Pisces  and  a  leg  in  Cetus, 
nor  should  13  Argos  pass  through  the  Unicorn's  flank  into  the  Little  Dog.  51  Oamelopar- 
dali  might  with  proiiriety  be  extracted  from  the  eye  of  Auripa,  a)!"!  the  ribs  of  Aquarius 
Released  froin  4C  Capricorni." 


210  THE  SIDEREAL  SYSTEM. 

however,  the  constellations  are  thus  rude  and  im- 
perfect, there  seems  little  hope  of  any  change.  Age 
gives  them  a  dignity  that  insures  their  perpetua- 
tion. 

The  Invention  of  the  Constellations  goes  back  into 
ages  of  which  no  record  remains.  By  some  it  has 
been  ascribed  to  the  Greeks.  When  the  signs  of  the 
zodiac  were  named,  they  doubtless  coincided  with 
the  constellations.  Aries  (the  ram)  was  so  called 
because  it  rose  with  the  sun  in  the  spring-time,  and 
the  Chaldean  shepherds  named  it  from  the  flocks, 
their  most  valued  possession.  Then  followed,  in 
order,  Taurus  (the  bull)  and  Gemini  (the  twins), 
called  from  the  herds,  which  were  esteemed  next  in 
value.  At  the  summer  solstice,  the  sun  appears  to 
stop,  and,  crab-like,  to  crawl  backward  ;  hence  the 
name  Cancer  (the  crab).  When  the  sun  is  in  Leo, 
the  brooks  being  dry,  the  lion  leaves  his  lurking- 
place  and  becomes  a  terror  to  all.  Virgo  comes 
next,  when  the  virgins  glean  in  the  summer  harvest. 
At  the  autumnal  equinox,  the  days  and  nights  are 
equally  balanced,  and  this  is  beautifully  represented 
by  Libra  (the  scales).  The  vegetation  decays  in  the 
fall,  causing  sickness  and  death  ;  the  Scorpion,  which 
stings  as  it  recedes,  is  suggestive  of  this  Parthian 
warfare.  Sagittarius  (the  archer)  tells  of  the  hunt- 
ing month.  Capricornus  (the  goat,  which  delights 
in  climbing  lofty  precipices)  denotes  how  at  the 
winter  solstice  the  sun  begins  to  climb  the  sky  on 
his  return  north.  Aquarius  (the  water-bearer)  is  a 
natural  emblem  of  the  rainy  season.  Pisces  (the 
fisHes)  is  the  month  for  fishing. 


THE  STAES.  211 

Signs  and  Constellations  do  not  Agree.  —  By  the 

precession  of  the  equinoxes,  as  we  have  before  de- 
scribed on  page  106,  the  signs  have  fallen  back  along 

Fig.  86. 


The  Signs  and  Constellations,  us  they  now  Compare  in  the  Heavens,  the  former  having 
fallen  back,  and  the  latter  apparently  advanced,  SO"  each. 

the  ecliptic  about  30°,  so  that  those  stars  which  were, 
during  the  infancy  of  astronomy,  in  the  sign  Aries 
{v)  are  now  in  Taurus  (a),  and  those  which  were  in 
the  sign  Pisces  (X)  are  now  in  Aries  {^)* 

Permanence  of  the   Constellations. — The  general 
appearance  of    the  constellations  and    the   figures 

*  If  the  teacher  will  put  a  pin  at  the  center  of  Fig.  86,  and  then  draw  a  sharp  knife 
between  the  signs  and  the  constellations,  so  as  to  detach  the  middle  of  the  cut,  and 
cause  the  inner  part  to  revolve,  the  signs  may  be  turned  before  any  constellation,  an(} 
|;hijs  this  change  be  clearly  apprehended. 


212  THE  SIDEREAL  SYSTEM. 

which  the  stars  form  are  due  to  the  position  we 
oecupy.  Could  we  cross  the  gulf  of  space  beyond 
Neptune,  the  stars  now  so  familiar  to  us  would  look 
strangely  enough  in  their  new  groupings.  As  one  in 
riding  through  a  forest  sees  the  trees  apparently 
increase  in  size  and  open  up  to  view  before  him. 
while  they  decrease  in  size  and  close  in  behind  him. 
forming  clusters  and  groups  which  constantly  change 
as  he  passes  along,  so,  as  our  earth  travels  -^vith  the 
solar  system  on  its  immense  sidereal  journey, 
the  stars  will  gradually  grow  larger  and  brighter  in 
front,  while  those  behind  us  will  appear  smaller  and 
dimmer. 

Since,  in  addition  to  this,  the  stars  themselves  are 
in  motion  with  varying  velocity  and  in  different 
directions,  the  constellations  must  change  still  more 
rapidly,  so  as  ultimately  to  transform  the  appear- 
ance of  the  heavens.  In  time,  the  "  Bands  of  Orion  " 
will  be  loosened,  and  the  ''  Seven  Sisters  "  will  glide 
apart.  Such  are  the  distances,  however,  that,  al- 
though these  movements  have  been  going  on  con- 
stantly, no  variation  has  occurred,  since  the  crea- 
tion of  man,  that  is  perceptible,  save  to  the  watchful 
astronomer.  Nothing  in  nature  is  so  invariable  as 
the  stars.  They  are  the  standards  of  time.  Myriads 
of  years  must  elapse  before  new  maps  of  the  constel- 
lations will  be  required. 

Value  of  the  Stars  in  Practical  Life.—"  The  stars 
are  the  landmarks  of  the  universe."  They  seem  to 
be  placed  in  the  heavens  by  the  Creator,  not  alone  to 
elevate  our  thoughts  and  expand  our  conceptions  of 
the  infinite  and  eternal,  but  to  afford  us,  amid  the 


THE   STARS.  313 

constant  fluctuations  of  our  own  earth,  something 
unchangeable  and  abiding.  Every  object  about  us 
is  constantly  shifting,  but  over  all  shine  the  "  eternal 
stars/'  each  with  its  place  so  accurately  marked, 
that  to  the  astronomer  and  the  geographer  no  decep- 
tion is  possible.  To  the  mariner,  the  heavens  be- 
come a  dial-plate,  the  figures  on  its  face  set  with 
glittering  stars,  along  which  the  moon  travels  as  a 
shining  hand  that  marks  off  the  hours  with  an  accu- 
racy no  watch  can  ever  rival.  Standing  on  the  deck 
of  his  vessel,  far  out  at  sea,  a  single  observation  of 
the  sun  or  the  stars  decides  his  location  in  the  waste 
of  waters  as  accurately  as  if  he  were  at  home,  and 
had  caught  sight  of  some  old  landmark  he  had 
known  from  his  boyhood.  In  all  the  intricacies  of 
surveying,  the  stars  furnish  the  only  immutable 
guide.  Our  clocks  vainly  strive  to  keep  time  with 
the  celestial  host.  Thus,  by  an  evident  plan  of  the 
Creator,  even  in  the  most  common  affairs  of  life,  are 
we  compelled  to  look  for  guidance  from  the  shifting 
objects  of  earth  up  to  the  heavens  above. 

Ancient  Views. — Anaximenes  (500  b.  c.)  thought 
that  the  stars  were  for  ornaments,  and  were  nailed 
like  bright  studs  into  the  crystalline  sphere.  Anaxa- 
goras  considered  that  they  were  stones  whirled  up 
from  the  earth  by  the  rapid  motion  of  the  ether,  and 
that  its  inflammable  properties  set  them  on  fire  and 
caused  them  to  shine  as  stars.  Some  schools  of  the 
Grecian  philosophers — the  Stoics,  Epicureans,  etc. — 
believed  that  they  were  celestial  fires  kept  alive 
by  matter  that  constantly  streamed  up  to  them  from 
the  center  of  the  heavens.     The  stars  were  at  one 


214  THE  SIDEREAL  SYSTEM. 

time  said  to  feed  on  air ;  at  another,  to  be  the  breath- 
ing holes  of  the  universe. 

Three  Zones  of  Stars. — If  we  recall  what  was  said 
on  page  90,  concerning  the  paths  of  the  stars  and  the 
appearance  of  the  heavens  at  different  seasons  of  the 
year,  we  shall  see  that  the  constellations  are  nat- 
urally divided  into  three  zones.  The  first  embraces 
those  which  are  visible  through  the  entire  year ;  the 
second,  those  whose  paths  can  be  seen  only  in  part 
on  any  given  night ;  and  the  third,  those  whose  paths 
just  graze  our  southern  horizon,  or  never  pass  above 
it. 


II.    THE   CONSTELLATIONS. 

I.  The   Northern  Circumpolar  Constellations  are 

visible  in  our  latitude  every  night.  They  may  be 
easily  traced  by  holding  the  book  up  toward  the 
northern  sky  in  such  a  way  that  Polaris  and  the  Big 
Dipper  on  the  map  and  in  the  heavens  agree  in  posi- 
tion, and  then  locating  the  other  constellations  by 
comparison. 

As  the  stars  revolve  about  Polaris,  their  places  will 
vary  with  every  successive  night  through  the  year. 
The  cut  represents  them  as  they  are  seen  at  midnight 
of  the  winter  solstice.  At  6  P.  M.  of  that  day,  the 
right-hand  side  of  the  map  should  be  held  downward, 
and  the  Big  Dipper  will  be  directly  below  the  north 
star.  At  6  a.  m.,  the  left-hand  side  should  be  at  the 
bottom,  and  the  Dipper  will  be  above  Polaris.    From 


THE  CIRCUMPOLAR   CONSTELLATIONS. 


215 


day  to  day,  this  aspect  will  change,  each  star  coming 
a  little  earlier  to  the  meridian,  or  to  its  position  on 
the  preceding  night.  The  rate  of  this  progression  is 
six  hours,  or  90°,  in  three  months. 

{Map  No.  1.)  Fig.  S7. 


^''.,it* 


.^      i   I    R  A    F    F    j,-      •  ^    \  ^ 


R        A 


Northern  Circumpotar  Constellations. 

Ursa  Major  is  represented  under  the  figure  of  a 
great  bear.  It  contains  133  stars  visible  to  the  naked 
eye.  This  constellation  has  been  celebrated  among 
all  nations.  It  is  remarkable  that  the  shepherds  of 
Chaldeain  Asia  and  the  Iroquois  Indians  of  America 
gave  to  it  the  same  name. 


216  THE  SIDEREAL  SYSTEM. 

Principal  Stars. — A  noticeable  cluster  of  seven 
stars — six  of  the  second  and  one  of  the  fourth  mag- 
nitude— forms  what  is  familiarly  termed  the  Dipper. 
In  England  it  is  styled  Charles's  "Wain,  from  a 
fancied  resemblance  to  a  wagon  drawn  by  three 
horses  tandem.  Mizar  {^)  has  a  ininute  companion, 
Alcor,  which  Humboldt  tells  us  could  be  rarely  seen 
in  Europe.  A  person  with  good  eyesight  may  now 
readily  detect  it.  Megrez  {6),  at  the  junction  of  the 
handle  and  the  bowl,  is  to  be  marked  particularly, 
since  it  lies  almost  exactly  in  the  colure  passing 
through  the  autumnal  equinox.  Dubhe  and  Merak 
are  termed  the  Pointers,  because  they  point  out  the 
polar  star.  The  bear's  right  fore-paw  and  hind-paw* 
are  each  marked  by  two  small  stars,  as  shown  in  the 
cut ;  a  similar  pair  nearly  in  line  with  these  denote 
the  left  hind-paw  (see  £,  Fig.  90). 

Mythological  History. — Diana  had  a  beautiful  attendant  named 
Callisto.  Juno,  the  queen  of  heaven,  becoming  jealous  of  the  maid,  trans- 
formed her  into  a  bear. 

"  The  prostrate  wretch  lifts  up  her  head  in  prayer, 
Her  arms  grow  shaggy,  and  deformed  with  hair  ; 
Her  nails  are  sharpened  into  pointed  claws. 
Her  hands  bear  half  her  weight  and  turn  to  paws. 
Her  lips,  that  once  would  tempt  a  god,  begin 
To  grow  distorted  in  an  ugly  grin. 
And  lest  the  supplicating  brute  might  reach 
The  ears  of  Jove,  she  was  deprivetl  of  speech. 
How  did  she  fear  to  loflge  in  woods  alone. 
And  haunt  the  fields  and  meadows  once  her  own  ! 
How  often  would  the  deep-mouthed  dogs  pursue, 
Whilst  from  her  hounds  the  frighted  hunters  flew." 

*  It  is  well  to  notice  that  Dubhe  and  Merak  are  about  5"  apart ;  Dubhe  and  Benet- 
nasch  are  about  25"  ajart ;  the  jmws  of  the  Beai  are  15"  apart ;  while  Polaris  is  about 
30°  distont. 


THE  CIRCUMPOLAR  CONSTELLATIONS.  21? 

Some  time  afterward,  CalJisto's  son,  Areas,  being  out  hunting,  pursued 
his  mother,  and  was  about  to  transfix  her  with  his  uplifted  spear,  when 
Jupiter  in  pity  transferred  them  both  to  the  heavens,  and  placed  them 
among  the  constellations  as  Ursa  Major  and  Ursa  Minor. 

Ursa  Minor  is  represented  under  the  figure  of  a 
small  bear.  It  contains  twenty-seven  stars,  of  which 
only  three  are  of  the  third,  and  four  of  the  fourth 
magnitude. 

Principal  Stars. — A  cluster  of  seven  stars  forms 
the  Little  Dipper.  Three  of  them  are  small,  and  are 
seen  with  difficulty.  Polaris,  at  the  extremity  of  the 
handle,  has  been  known  from  time  immemorial  as 
the  North  Polar  Star.  Until  the  mariners  compass 
came  into  use,  it  was  the  star 

"  Whose  faithful  beams  conduct  the  wandering  ship 
Through  the  wide  desert  of  the  pathless  deep." 

Polaris  does  not  mark  the  exact  position  of  the  pole, 
since  that  is  about  1^°  toward  the  Pointers.  This 
distance  will  gradually  diminish,*  until  in  time 
(2130  A.  D.)  it  will  be  only  |° :  then  it  will  increase 
again,  until,  in  the  lapse  of  ages,  12,000  years  hence, 
the  brilliant  star  Vega  (a  Lyrse)  will  fulfill  the  office  of 
polar  star  for  those  who  shall  then  live  on  the  earth,  f 
The  Distance  of  Polaris  is  so  great,  that,  though 
the  star  is  moving  through  space  at  the  rate  of  ninety 

*  Five  stars  of  the  Dipper  itself  are  drifting  away  from  tlie  sim,  at  tlie  rate  of  17 
miles  per  second,  .seeming  to  form  a  family  or  group  by  themselves.  Proctor's  Easy 
Star  Lessons  gives  charts  representing  the  appearance  of  the  Dipper  for  100,000  years. 

\  Of  the  nine  Pyramids  which  are  standing  at  Gizeh,  Egypt,  six  have  openings  facing 
the  north.  These  lead  to  straight  passages  which  descend  at  a  uniform  angle  of  about 
26°  and  are  parallel  with  the  meridian.  If  we  supi)o.se  a  person,  4,000  years  ago,  stand- 
ing at  the  lower  end  of  one  of  these  passages,  and  looking  out,  his  eye  would  strike  the 
sky  near  the  st:ir  Tliuban,  which  was  then  the  jiolar  star.  The  supposed  date  of  the 
building  of  these  Pyramids  (the  Great  Pyramid,  2123  b.  c.)  agrees  with  that  epoch, 
and  naturally  suggests  that  the  builders  had  some  special  design  in  this  peculiar  con- 
struction. 


218  THE   SIDEREAL   SYSTEM. 

miles  per  minute,  this  tremendous  speed  is  impercep- 
tible to  us.  It  requires  nearly  fifty  years  for  its  light 
to  reach  the  earth ;  so  that,  when  we  look  at  Polaris, 
we  know  that  the  ray  which  strikes  our  eye  set  out 
on  its  journey  through  space  half  a  century  ago. 
We  cannot  state  positively  that  the  star  is  now  in 
existence,  since  if  it  were  destroyed  to-day  it  would 
be  fifty  years  before  we  should  miss  it.  * 

Calculation  of  Latitude  from  Polaris.— By  an 
observer  at  the  equator,  Polaris  is  seen  at  the  horizon. 
If  he  goes  north,  the  horizon  is  depressed,  and  Polaris 
seems  to  rise  in  the  heavens.  When  it  has  reached 
the  height  of  a  degree,  the  observer  is  said  to  have 
passed  over  a  degree  of  latitude  on  the  earth's  sur- 
face. As  he  moves  further  north,  the  polar  star  con- 
tinues to  ascend  ;  its  distance  above  the  horizon 
denoting  the  latitude  of  each  place  in  succession, 
until  at  the  north  pole,  if  one  could  reach  that  point, 
Polaris  would  be  seen  directly  overhead. 

Drftco  is  represented  under  the  figure  of  a  long 
sinuous  serpent,  stretching  between  Ursa  Major  and 
Ursa  Minor,  nearly  encircling  the  latter  constellation, 
and  finally  reaching  out  its  head  almost  to  the  body 
of  Hercules. 

Principal  Stars. — Four  small  stars  form  a  quad- 
rilateral figure  at  the  head ;  a  fifth,  of  the  fourth 
magnitude,  which  is  scarcely  visible,  marks  the  end 
of  the  nose ;  several  scattered  groups  and  little 
triangles  of  small  stars  denote  the  position  of  the 
various  coils  of  the  body ;  thence,  an  irregular  line 
of  stars  traces  the  dragon's  tail  around  between  Ursa 

*  Some  recent  observations  seem  to  reduce  this  to  42  years. 


THE  CIUCUMPOLAR   constellations.  219 

Major  and  Ursa  Minor.  Tliuban,  lying  midway  be- 
tween y  of  the  Little  Dipper  and  ^  of  the  Big  Dipper, 
is  noted  as  the  polar  star  of  forty  centuries  ago. 

Mythological  History. — Jupiter  had  carried  off  Europa.  7\genor, 
her  father,  sent  her  brother  Cadmus  in  pursuit  of  his  lost  sister,  bidding 
him  not  to  return  until  he  was  successful  in  his  search.  After  a  time, 
Cadmus,  weary  of  his  wanderings,  inquired  of  the  oracle  of  Apollo  concern- 
ing the  fate  of  Europa.  He  was  told  to  cease  looking  for  his  sister,  to  fol- 
low a  cow  as  a  guide,  and  when  she  rested,  there  to  build  a  city.  Hardlj' 
had  Cadmus  stepped  out  of  the  temple,  when  he  saw  a  cow  slowly  walking 
along.  He  followed  her  until  slie  came  upon  the  broad  plains  where 
Thebes  afterward  stood.  Here  she  stopjied.  Cadmus,  wishing  to  offer  a 
sacrifice  to  Jupiter  in  gratitude  for  the  delightful  location,  sent  his  servants 
to  seek  for  water.  In  a  dense  grove  near  by,  was  a  fountain  guarded  by  a 
fierce  dragon  {Draco),  and  sacred  to  JIars.  The  Tyrians,  approaching  this 
and  attempting  to  dip  up  some  water,  were  attacked,  and  many  of  them 
killed,  by  the  enormous  serpent,  whose  head  overtopped  the  tallest  trees. 
Cadmus,  becoming  impatient,  went  in  search  of  his  men,  and,  on  arriving 
at  the  spring,  saw  the  sad  disaster.  He  forthwith  fell  upon  the  mon- 
ster, and  after  a  severe  battle  succeeded  in  slaying  him.  While  standing 
over  his  conquered  foe,  he  heard  a  voice  from  the  ground  bidding  him  take 
the  dragon's  teeth  and  sow  them.  He  obeyed.  Scarcely  had  he  finished 
when  the  earth  began  to  move  and  the  points  of  spears  to  prick  through 
the  surface.  Next,  nodding  plumes  shook  off  the  clods,  and  the  heads  of 
armed  men  protruded.  Soon  a  great  harvest  of  warriors  covered  the  entire 
plain.  Cadmus,  in  terror  at  the  appearance  of  those  giants,  whom  he 
termed  Sparti  {the  soion),  prepared  to  attack  them,  when  suddenly  they 
turned  upon  themselves,  and  never  ceased  their  warfare  till  only  five  of 
the  crowd  survived.  These,  making  peace  with  one  another,  joined  Cadmus, 
and  assisted  him  in  building  the  City  of  Thebes. 

Cepheus  is  represented  as  a  king  in  regal  state, 
with  a  crown  of  stars  on  his  head,  while  he  holds  in 
his  hand  a  scepter  which  is  extended  toward  his  wife, 
Cassiopeia.  The  constellation  contains  thirty-five 
stars  visible  to  the  naked  eye. 


220  THE  SIDEREAL   SYSTEM. 

Principal  Stars. — The  brightest  star  is  Alderamin 
(a),  in  the  right  shoulder.  Alphirk  (;^),  in  the  girdle,  is 
at  the  common  vertex  of  several  triangles,  which  point 
out  respectively  the  left  shoulder  (i),  the  left  knee  (y), 
and  the  right  foot.  The  head,  which  lies  in  the  Milky 
Way,  is  marked  by  a  little  triangle  of  three  stars. 
This  forms,  with  a,  (3,  and  i,  quite  a  regular  quad- 
rilateral figure.  A  bright  star  of  the  fifth  magnitude, 
close  to  Polaris,  points  out  the  left  foot. 

Cassiopeia^  is  represented  as  a  queen  seated  on 
her  throne.  On  her  right,  is  the  king ;  on  her  left, 
Perseus,  her  son-in-law ;  above  her,  Ajidromeda, 
her  daughter.  The  constellation  contains  sixty-seven 
stars  visible  to  the  naked  eye. 

Principal  Stars. — A  line  drawn  from  Megrez  ((J), 
in  Ursa  Major,  through  Polaris  and  continued  an 
equal  distance,  will  strike  Caph  {13)  in  Cassiopeia. 
This  star  is  noticeable  as  marking,  with  the  others 
named,  the  equinoctial  colure,  and  as  being  on  the 
same  side  of  the  true  pole  as  Polaris.  The  principal 
stars  form  the  figure  of  an  inverted  chair,  which  is 
very  striking  and  may  be  easily  traced. 

II.  Equatorial  Constellations. — The  constellations 
we  shall  now  describe  lie  south  of  the  circumpolar 
groups.  Only  a  portion  of  their  paths  is  above  our 
horizon.  In  using  the  maps,  the  observer  is  supposed 
to  stand  with  his  back  toward  Polaris,  and  to  be  look- 
ing directly  south.  Commencing  with  the  constella- 
tion Perseus,  so  intimately  connected  with  the  other 

*  Fo»  tlie  mythological  history  of  Cassiopeia,  see  Perseus  and  Andromeda.  The  names 
of  the  principal  stars  in  the  Chair  make  a  mnemonic  word  ,—0(iy  ^e,  hagdc.  The  student 
can  often  form  such  an  association  of  the  letters,  and  will  fiud  the  device  an  aid  to  his 
memory.    (Compare  Virgo,  page  230.) 


EQUATORIAL  CONSTELLATIONS.  221 

members  of  the  royal  family  just  described,  we  pass 
eastward  in  our  survey,  and  notice  the  various  con- 
stellations as  they  slowly  defile  in  silent  march  across 
the  sky. 

The  first  map  represents  the  constellations  on  or 
near  the  meridian  at  nine  o'clock  in  the  evening-  of 
the  winter  solstice.  On  the  evening-  of  the  autumnal 
equinox,  the  left-hand  side  of  the  map  should  be 
turned  downward  toward  the  eastern  horizon.  On 
the  evening  of  tlie  vernal  equinox,  the  right-hand 
side  should  be  turned  to  the  western  horizon.  At 
these  different  times,  the  stars,  though  keeping 
their  relative  positions,  will  be  diversely  inclined  to 
the  horizon.  As  the  stars  apparently  move  westward 
at  the  rate  of  lo""  per  hour,  the  time  of  the  evening 
determines  what  stars  will  be  visible,  and  also  their 
distances  above  the  horizon. 

Perseus  is  represented  as  brandishing  an  enor- 
mous sword  in  his  right  hand,  while  in  his  left  he 
holds  the  head  of  Medusa.  The  constellation  com- 
prises eighty-one  stars  visible  to  the  naked  eye. 

Principal  Stars. — The  most  prominent  figure  is 
called  the  Segment  of  Perseus.  It  consists  of  several 
stars  arranged  in  a  line  curving  toward  Ursa  Major. 
Algenib  (a),  the  brightest  of  these,  is  of  the  second 
magnitude.  Algol  (p.  242),  in  the  midst  of  a  group  of 
small  stars,  marks  the  head  of  Medusa.  Between  the 
bright  stars  of  Perseus  and  Cassiopeia,  is  a  beautiful 
star-cluster  visible  to  the  naked  eye. 

MythologicaIj  HisToiiY. — Perseus,  from  whom  this  constellation  was 
named,  was  the  son  of  Jupiter  and  Danaii.  His  grandfather,  Acrisius,  liav- 
ing  been  iuformed  by  the  oracle  that  his  grandson  would  be  the  instru- 


3XJ2 


THE  SIDEREAL  SYSTEM, 
(Map  No.  8}— Fig.  88. 


Aj\-D  R  o■^^^ 


f    I    S    H  ,E-S 


C  n      ^,,«. 


ment  of  his  death,  put  the  mother  and  child  in  a  coffer  and  set  them  adrift 
on  the  sea.  Fortunately,  they  floated  near  tlie  island  Seriphus,  where  they 
were  rescued  and  kindly  treated  by  a  brother  of  Polydectes,  king  of  the 
country.  "When  Perseus  had  grown  up,  he  was  ordered  by  Polydectes 
to  bring  him,  as  a  marriage  gift,  the  head  of  Medusa.  Xow  Medusa  was 
once  a  beautiful  maiden,  who  dared  to  compare  her  ringlets  with  those  of 
Minerva  ;  whereupon,  the  goddess  changed  her  locks  into  hissing  serpents, 
and  made  her  features  so  hideous,  that  she  turned  to  stone  every  living 
object  upon  which  she  lixed  her  Gorgon  gaze.  Perseus  was  at  first  over- 
powered at  the  thought  of  undertaking  this  enterprise;  but  Mercury 
promised  to  be  his  guide,  and  to  furnish  him  with  his  winged  .shoes ;  Mi- 
nerva loaned  him  her  wonderful  shield,  that  was  bright  as  a  mirror ;  and 
the  Nymphs  gave  him,  in  addition,  Pluto's  helmet,  which  made  the  bearer 
invisible.  Thus  equipped,  Perseus  mounted  into  the  air  and  flew  to  the 
ocean,  where  he  found  the  three  Gorgons,  of  whom  Medusa  was  one,  asleep. 
Fearing  to  gaze  in  her  face,  he  looked  upon  tlie  image  reflected  in  Minerva's 
shield,  and  with  his  sword  severed  Medusa's  head  from  her  body.  The 
blood  gushed  forth,  and  with  it  the  winged  steed  Pegasus.  Grasping  the 
head,  Perseus  flew  away.  The  other  Gorgons  awaking,  pursued  him,  but 
he  escaped  their  search  by  means  of  Pluto's  helmet    As  he  flew  over  the  wilds 


EQUATORIAL    CONSTELLATIONS.  223 

of  Libya,  in  his  aerial  route,  the  blood  dripping  from  the  gory  head  of  the 
monster  produced  the  innumerable  serpents  for  which  that  country  was 
afterward  noted. 

Andromeda  is  represented  as  a  beautiful  maiden 
chained  to  a  rock. 

Principal  Stars. — Algenib  and  Algol  in  Perseus 
form,  with  Almach  (y)  in  the  left  foot  of  Andromeda, 
a  riglit-angled  triangle  opening  toward  Cassiopeia. 
This  figure  is  so  perfect,  that  the  stars  may  be  easily 
recognized.  The  girdle  is  pointed  out  by  Merach  (/3), 
and  two  other  stars  which  form  a  line  slightly  curv- 
ing toward  the  right  foot.  The  breast  is  denoted  by 
a  very  small  triangle  composed  of  three  stars, — 6  of 
the  fourth  magnitude,  another  of  the  fifth  magnitude 
just  south,  and  an  exceedingly  minute  star  a  little  at 
the  west.  Alpheratz  («),  in  the  head  of  Andromeda, 
belongs  also  to  Pegasus.  This  star,  with  three  others 
—Algenib  (y),  Markab  (a),  and  Scheat  (^3),— all  of 
the  second  magnitude,  constitute  the  Great  Square 
of  Fegasus.  The  brightest  stars  of  these  two  con- 
stellations form  a  figure  strikingly  like  the  Big  Dip- 
per. Algenib  and  Alpheratz  lie  in  the  equinoctial 
colure  which  passes  through  Caph. 

Mythological  History.  —Cassiopeia  had  boasted  that  her  daughter 
Andromeda  was  fairer  than  the  Sea-nymphs.  They  appealed,  in  great 
indignation,  to  Neptune,  who  sent  a  sea-monster  {Cetus)  to  devastate  the 
coast  of  Ethiopia.  To  appease  the  deities,  her  father  Cepheus  was  directed 
by  the  oracle  to  bind  his  daughter  to  a  rock,  to  be  devoured  by  Cetus. 
Perseus,  returning  from  the  destruction  of  Medusa,  saw  Andromeda  in  her 
forlorn  condition.  Struck  by  her  beauty  and  tears,  he  offered  to  liberate 
her  at  the  price  of  her  hand.  Her  parents  joyfully  consented,  and,  in 
addition,  offered  a  royal  dower.     Perseus  slew  the  terrible  monster,  and. 


^24  THE    SIDEREAL  SYSTEM. 

freeing  Andromeda,  restored  her  to  hsr  parents.  All  the  promiuent  actors 
in  this  scene  were  honored  with  seats  among  the  constellations.  The  Sea- 
nymphs,  it  is  said,  in  petty  spite  of  Casoioj>eia,  prevailed  that  she  should 
be  placed  where  for  half  of  the  time  she  hangs  with  her  head  do^v-nward, — a 
fit  lesson  of  humility.     Cephcus,  her  husband,  shares  in  her  punishment. 

Aries,  the  irim,  was  anciently  the  first  constella- 
tion of  the  zodiac.  It  is  now  the  first  sign,  but  the 
second  constellation.  On  account  of  the  precession 
of  the  equinoxes,  the  constellation  Pisces  occupies 
the  first  sign. 

PRI^'CIPAL  Stars. — The  most  noted  star  is  a  Arietis 
(Alpha  of  Aries,  more  commonly  called  simply  Arie- 
tis), in  the  right  horn.  This  lies  near  the  path  of  the 
moon  and  is  one  of  the  stars  from  which  longitude  is 
reckoned.  A  line  di'awn  from  Almach  to  Arietis 
will  pass  through  a  beautiful  figure  of  three  stars 
called  TJie  Triangles. 

Mythological  History. — Phrixns  and  Helle  were  the  children  of 
Athamas,  king  of  Thessalj*.  Being  persecuted  by  Ino,  their  step-mother, 
they  were  com}>elled  to  flee  for  safety,  ilercury  provided  them  a  ram 
which  bore  a  golden  fleece.  The  children  were  no  sooner  placed  on  his  back 
than  he  vaulted  into  the  heavens.  In  their  aerial  jouniey,  Helle  becoming 
dizzy  fell  off  into  the  sea,  which  was  afterward  called  the  Hellespont,  now 
the  Dardanelles,  Phrixus  having  reached  Colchis  in  safety,  offered  the  ram  in 
sacrifice  to  Jupiter,  and  gave  the  golden  fleece  to  Aetes,  his  protector.  The 
Argonautic  exjiedition  in  pursuit  of  this  golden  fleece,  by  Jason  and  his 
followers,  is  one  of  the  most  romantic  of  mythological  stories.  It  is, 
undoubtedly,  a  fanciful  account  of  the  first  important  maritime  expedition. 
Rich  spoils  were  the  prizes  to  be  secured. 

Taurus  consists  of  the  head  and  shoulders  of  a 
bull,  which  is  represented  in  the  act  of  plunging  at 
Orion. 

Principal  Stars. — The  Hyades,  a  beautiful  cluster 


EQUATORIAL   CONSTELLATIONS.  225 

in  the  head,  forms  a  distinct  V.  The  brightest  of 
these  is  Aldebaran,  a  fiery  red  star  of  the  first  mag- 
nitude.* The  Pleiades  (Job,  xxxviii,  31),  or  the  Seven 
Sisters,  is  the  most  conspicuous  group  in  the  sky 
(p.  206).  It  contains  a  large  number  of  stars,  six  of 
which  are  visible  to  the  naked  eye.  There  were  said 
to  have  been  seven  anciently,  but  that  Electra  left 
her  place  in  order  not  to  behold  the  ruin  of  Troy, 
which  was  founded  by  her  son  Dardanus.  Other 
myths  relate  that  the  ''Lost  Pleiad''''  was  Merope, 
who  married  a  mortal.  Alcyone  is  the  brightest 
Pleiad.  El  Nath  (r^)  and  ^  point  out  the  horns  of 
Taurus. 

Mythological  Hlstoiiy. — This  is  the  animal  whose  form  Jupiter 
assumed  when  he  bore  off  Europa.  The  Pleiack-s  were  the  daughters  of 
Atlas,  and  Nymphs  of  Diana's  train.  They  were  distinguished  for  their 
unblemished  virtue  and  mutual  affection.  The  hunter  Orion  having  pur- 
sued them  one  day,  iu  their  distress  they  prayed  to  the  gods,  when  Jupiter, 
in  pity,  transferred  them  to  the  heavens. 

Aurlf/a,  the  Charioteer  or  Wagoner,  is  represented 
as  a  man  resting  one  foot  on  a  horn  of  Taurus,  and 
holding  a  goat  and  kids  in  his  left  hand  and  a  bridle 
in  his  right. 

The  Principal  Stars  are  arranged  in  an  irregular 
five-sided  figure.  Capella,  the  goat-star,  is  of  the 
first  magnitude.  It  travels  in  its  orbit  1,800  miles  per 
minute  ;  seventy  years — a  long  lifetime —are  required 
for  its  light  to  reach  the  earth.  Near  by  is  a  tiny 
triangle,  formed  of  three  small  stars,  called  the  Kids. 
Menkalinan  (,'?)  is  in  the  right  shoulder,  d  in  the  right 

*  Aldebaran  is  estimated  to  move  through  the  heavens  at  tlic  rate  of  55  miles  per 
second.  (See  pp.  205,  201.)  A  numl)er  of  the  liright  stars  between  Aldel^aran  and  the 
Pleiades  have  a  common  motion  of  alxjut  10"  per  century  toward  the  east. 


226  THE   SIDEREAL   SYSTEM. 

hand,  /3  (common  to  Auriga  and  Taurus)  the  right 
foot,  and  i  the  left  foot.  Capella,  /3,  and  6  (a  star  in 
the  head)  form  a  triangle.  The  origin  of  this  con- 
stellation is  unknown. 

I^isces,  the  fishes,  is  represented  by  two  fishes  tied 
together  bv  a  long  ribbon.  It  consists  of  small  stars, 
which  can  be  traced  only  upon  a  clear  night,  and  in 
the  absence  of  the  moon. 

CefKS,  the  ichale,  is  a  huge  sea-monster,  slowly 
ploughing  his  way  eastward,  midway  between  the 
horizon  and  the  zenith.  It  may  easily  be  found,  on 
a  clear  night,  by  means  of  the  numerous  figures 
given  in  the  map. 

(Map  So.  S)—Fxg.  S9. 


" 

lUu^'fs            .    n       .      ^ 

. 

.    -U    R  1   G   A 

.Ram 

Castor  •- 

'"•*'" 

;'  """"---  ^ 

•      ;  »      -^'U,^      -L;' 

L                             /U.' 

Polhi  .     ■■• 

•                                • *     ^^^      '  .' 

,-*r---- 

■       .^~.^T     W    I 

,     S                           '  '     '      . 

'"7^~~~ 

l^lTTLE.   Dot 
Pj'ocyon 

< 

■-J     D                   *~"                                  "f- 

* 

Sirius     ■        /\^ 

\                                                                 V  ' 

HARE 

1                           a 

^"^AT,, 

.''- 

•  .^•     ''og 

^c- 

%6       *c. 

;>::t;    .  ;      _  .^* 

,/>  • 

- 

Gemini^  the  Ticins,  represents  the  twin  brothers 
Castor  and  Pollux. 

•  "  Castor  is  resolved  by  the  telescope  into  two  stars,  whose  angular  distance  from 
each  other  is  5"— the  angle  that  one  inch  would  subtend  1,146  j-ards  off."— Ball.    ' 


EQUATORIAL   CONSTELLATIONS.  227 

The  Principal  Stars  are  Castor'-'  and  Pollux,  which 
are  of  the  first  and  second  magnitudes.  The  latter 
is  one  of  the  stars  from  which  longitude  is  reckoned 
by  means  of  the  Nautical  Almanac.  The  constella- 
tion is  clearly  distinguished  by  two  nearly  parallel 
rows  of  stars,  that  by  a  slight  effort  of  the  imagina- 
tion may  be  extended  into  the  constellations  Taurus 
and  Orion. 

Mythological  History.  — Castor  and  Pollux  were  noted, — the  former 
for  his  skill  in  training  horses,  the  latter  for  boxing.  They  were  tenderly 
attached  to  each  other,  and  were  inseparable  in  their  adventures.  They  ac- 
companied Jason  on  the  Argonautic  expedition.  A  storm  having  arisen 
during  this  voyage,  Orpheus  played  on  his  wonderful  lyre  and  prayed  to  the 
gods ;  whereupon  the  tempest  was  stilled,  and  star-like  flames  shone  upon 
the  heads  of  the  twin-brothers.  Sailors,  therefore,  considered  them  as 
patron  deities,*  and  the  balls  of  electric  flame  seen  on  masts  and  shrouds, 
now  called  St.  Elmo's  fire,  were  named  after  them.  Afterward,  Castor  was 
slain.  Pollux  being  inconsolable,  Jujuter  off'ered  either  to  take  him  up  to 
Olympus,  or  to  let  him  share  his  immortality  with  his  brother.  Pollux  pre- 
ferred the  latter,  and  so  tlie  brothers  pass  alternately  one  day  under  the 
earth,  and  the  next  in  the  Elysian  Fields.  Not  only  did  sailors  thus  con- 
fide in  their  watch  over  navigation,  but  soldiers  believed  them  to  return, 
mounted  on  snow-white  steeds  and  clad  in  rare  armor,  to  take  part  in  the 
hard-fought  battle-fields  of  the  Romans. 

"  Back  comes  tlie  chief  in  triumph, 
Who  in  the  hour  of  flglit 
Hath  seen  the  great  Twin  Brethren, 

In  harness  on  his  right. 
Safe  comes  the  ship  to  haven, 

Through  billows  and  through  gales. 
If  once  the  great  Twin  Brethren 
Sit  shining  on  the  sails."— Lays  of  Ancient  Rome. 

Orion  is  represented  under  the  figure  of  a  hunter 
assaulting  Taurus.     He  has  a  sword  in  his  belt,  a 

*  We  remeinbci-  that  Paul  saileil  for  Italy  in  a  ship  whose  sign  was  Castor  and  Pollux. 
■^Acts,  xxviii.  11. 


228  THE   SIDEREAL  SYSTEM. 

club  in  his  right  hand,  and  the  skin  of  a  lion  in  his 
left.  This  is  one  of  the  most  clearly  defined  and  con- 
spicuous constellations  in  the  heavens. 

Principal  Stars. — Four  brilliant  stars,  in  the  form 
of  a  parallelogram,  mark  the  outlines  of  Orion.  Betel- 
geuse,  a  beautiful  ruddy  star  of  the  first  magnitude, 
is  in  the  right  shoulder;  Bellatrix  (>),  of  the  second 
magnitude,  is  in  the  left  shoulder ;  Rigel,  of  the  first 
magnitude,  is  in  the  left  foot ;  and  Saiph  (>«),  of  the 
third  magnitude,  is  in  the  right  knee.  Two  small 
stars  near  /-  form  with  it  a  small  triangle,  which  is 
itself  the  vertex  of  a  larger  triangle  composed  of  /-, 
•/,  and  Betelgeuse.  Near  the  center  of  the  parallel- 
ogram are  three  stars  forming  the  Belt  of  Orion. 
This  group  is  also  called  the  Bands  of  Orion  (Job, 
xxxviii,  31),  Jacob's  rod,  and  the  Yard.  It  received 
the  last  name  because  it  forms  a  line  3'  long,  divided 
in  equal  parts  by  a  star  in  the  center.  These  divi- 
sions are  useful  for  measuring  the  distances  of  the 
stars.  Running  from  the  belt  southward,  is  an 
irregular  line  of  stars  which  marks  the  sword  ;  west 
of  Bellatrix  is  a  curved  line  denoting  the  lion's  skin. 
South  of  Orion  are  four  stars  forming  a  beautiful 
figure  styled  The  Hare, 

Mythological  Histohy. — Orion  was  a  famous  hunter.  Becoming 
enamored  of  Merope,  he  desired  to  marry  her.  (Enopion,  her  father, 
opposing  the  choice,  put  out  the  eyes  of  the  unwelcome  suitor.  The 
blinded  hero  followed  the  sound  of  a  Cyclop's  hammer  until  he  came  to 
Vulcan's  forge.  Vulcan,  taking  pity,  instructed  Kedalion  to  conduct  him 
to  the  abode  of  the  sun.  Placing  his  guide  on  his  shoulder,  Orion  pro- 
seeded  to  the  east,  and  at  a  favorable  place 

"  Clinibint;  up  a  narrow  gorge. 
Fixed  his  blank  eyes  upon  the  sun." 


EQUATORIAL  CONSTELLATIONS.  229 

The  healing  beams  restored  him  to  sight.  As  a  punishment  for  having 
profanely  boasted  that  he  was  able  to  conquer  any  animal  the  earth  could 
produce,  he  was  bitten  in  the  heel  by  a  scorpion.  Afterward,  Diana  placed 
him  among  the  stars ;  where  Sirius  and  Procyon,  liis  dogs,  follow  liim,  the 
Pleiades  fly  before  him,  and  far  remote  is  the  Scorj/km,  by  whose  bite  he 
perished. 

Canis  Major  and  Ca^iis  Minor  contain  each  a 
single  star  of  the  first  magnitude,  Sirius,  and  Pro- 
cyon.* These  two,  with  Betelgeuse,  Phaet  in  the 
Dove,  and  Naos  in  the  Ship,  form  a  huge  figure 
known  as  the  Egyptian  X.  Sirius,  the  dog-star,  is 
the  most  brilliant  star  in  the  heavens.  It  is  reced- 
ing from  the  earth  at  the  rate  of  20  miles  per  second 
(Huggins).  Seventeen  years  are  required  for  its  light 
to  reach  us.f     (See  note,  p.  308.) 

Leo  is  represented  as  a  rampant  lion.  It  is  one  of 
the  most  beautiful  constellations  in  the  zodiac. 

The  Principal  Stars  are  arranged  in  the  form  of 
a  sickle.  Regulus,  in  the  handle,  is  a  brilliant  star  of 
the  first  magnitude.  It  is  one  of  the  stars  from 
which  longitude  is  reckoned.  It  is  almost  exactly  in 
the  ecliptic.  Zosma  {6)  lies  in  the  back  of  the  lion, 
6  in  the  thigh,  and  Denebola,  a  star  of  the  second 
magnitude,  in  the  brush  of  the  tail. 

Cancer  includes  the  stars  that  lie  irregularly 
scattered  between  Gemini,  Head  of  Hydra,  Procyon, 
and  Leo.  In  the  midst  of  these,  is  a  luminous  spot, 
called  Praesepe,  or  the  Bee-hive,  which  an  ordinary 
glass  will  resolve  into  stars. 

*  Procyon,  like  Sirius,  was  formerly  considered  a  star  of  evil  omen,  and  as  bringing 
bad  weather.  "  Who  that  is  learned  in  matters  astronomioal,"  said  Digges,  the  astrol- 
oger, "  noteth  not  the  great  efl'ects  at  the  rising  of  the  star  called  the  Litel  Dogge." 

t  In  1802.  Alvan  G.  Clark,  son  of  the  famous  telescope-maker,  discovered  a  companion 
of  Sirius,  "distant  from  the  star  28  times  the  Sun's  distance  from  the  Earth." 


230  THE  SIDEREAL  SYSTEM. 

Virgo  is  represented  as  a  beautiful  maiden  with 
folded  wings,  bearing  in  her  left  band  an  ear  of  corn. 

The  Pri>-cipal  Stae,  Spica,  in  the  ear  of  corn,  is  of  ^ 
the  first  magnitude,  and  is  used  for  determining  Ion-- 
gitude  at  sea.  Denebola,  Cor  Caroli  (a),  Arcturus, 
and  Spica  form  a  figure  about  50=  in  length,  called 

(Map  No.  Uy  Fig.  00. 


the  Diamond  of  Virgo.  Five  third-magnitude  stars, 
e  6  y,  7],  .^,  (the  mnemonic  word  is  begde)  make  a 
corner  known  among  the  Arabian  astronomers  as 
"  The  retreat  of  the  howling  dog." 

Mythological  History. -Virgo  was  the  Goddess  Astr^a.  According 
to  the  poets,  the  early  history  of  man  ..as  the  golden  age.  It  was  a  time 
of  innocence  and  truth.  The  gods  dwelt  among  men,  and  perpetual  spring 
delighted  the  earth.     Next,  came  the  sUver  age,  less  tranquil  and  serene. 


EQUATORIAL  CONSTELLATIONS.  231 

but  still  the  gods  lingered  and  happiness  prevailed.  Then  followed  the 
brazen  and  iron  ages,  when  wickedness  reigned  supreme.  The  earth  was 
wet  with  slaughter.  The  gods  left  the  abodes  of  men,  one  by  one,  Astrsea 
alone  remaining  ;  until  finally  she  too,  last  of  all  the  immortals,  bade 
the  earth  farewell.     Jupiter  thereupon  placed  her  among  the  constellations. 

Hifdra  is  a  long,  straggling  serpent,  having  its 
head  near  Procyon  and  extending  its  tail  beyond 
Virgo,  a  total  distance  of  more  than  100°. 

The  Principal  Star  is  Cor  Hydrse,  of  the  second 
magnitude.  It  is  a  lone  star,  and  may  be  easily 
found  by  a  line  drawn  from  y  Leonis  through  Regu- 
lus,  and  continued  about  23°.  The  head  is  marked 
by  a  rhomboidal  figure  of  four  stars  of  the  fourth 
magnitude  lying  near  Procyon.  Several  little 
triangles  may  be  formed  of  them  and  other  small 
stars  lying  near.  The  Crater,  or  Cup,  is  a  beautiful 
and  very  striking  semicircle  of  six  stars  of  the  fourth 
magnitude  directly  south  of  0  Leonis.  Corvus  {S,  e, 
y,  (5),  the  raven,  lies  15°  east  of  the  Cup.  e  Corvi  is  in 
the  equinoctial  colure. 

Mythological  History. — Hydra  was  a  fearful  serpent  which  in  ancient 
times  infested  the  lake  Lerna.  Its  destruction  constituted  one  of  the 
twelve  labors  of  Hercules.  Tlie  Crow  was  formerly  white,  it  is  said,  but 
was  changed  to  its  raven  tint  on  account  of  its  proneiiess  to  tale-bearing. 

Canes  Vetiaticlf  the  hunting  dogs.  This  constel- 
lation contains  the  bright  star.  Cor  Caroli  (a),  which  is 
found  by  a  line  passing  from  Benetnasch  (??)  through 
Berenice's  Hair  to  Denebola  (/3). 

Berenice's  ILiir  is  a  beautiful  cluster  midway 
between  Cor  Caroli  and  Denebola.  Near  by  is  a 
single  bright  star  of  the  fourth  magnitude. 


232 


THE  SIDEREAL  SYSTEM. 


Mythological  History.— Berenice  was  the  wife  of  Ptolemy.  Her 
husban.l  going  upon  a  dangerous  expedition,  she  promised  to  consecrate 
her  beautiful  tresses  to  Venus  if  he  should  return  in  safety.  Soon  after  the 
fulfilment  of  this  vow  the  hair  disappeared  from  the  temple  where  it  had 
been  deposited.  Berenice  being  much  disquieted  at  this  loss,  Conon,  the 
astronomer,  announced  that  tlie  locks  had  been  transferred  to  the  heavens, 
in  proof  of  which  he  pointed  out  this  cluster  of  hitherto  unnamed  stars. 
All  parties  were  satisfied  with  this  happy  termination  of  the  difficulty. 

Bootes,  the  bear-driver,  is  represented  as  a  hunts- 
man grasping  a  club  in  his  right  hand,  while  in  his 

(Map  No.  Sy-Fig.  91. 


R   E    A'  T     ,-'B   E 


^   'c     0  .  ;   "V  JO  • 


g  •     BERENICE' „       .     .      \ 

li     ■^'''  '        ':■.  8«  *L  1  0  N.." 

-7  t        0  .•  .        '  \  -'-' 

~-   ^.  ^         .       .      ,>''  e 

•:  .  ^\  >    •  •  •  / 


left  he  holds  by  the  leash  his  two  greyhounds  {Canes 
Venatici),  with  which  he  is  pursuing  the  Great  Beai 
continually  around  the  north  pole. 

Principal  Stars.— Arcturus  (Job,  ix,  9),  a  mag- 
nificent  star  of  the  first  magnitude,  is  in  the  left 
knee.  It  forms  a  triangle  with  Denebola  and  Spica, 
and  also  one  with  Denebola  and  Cor  Caroli.  It 
travels  in  its  orbit  fifty-five  miles  per  second,   qv 


EQUATORIAL  CONStELLAflONg.  333 

three  times  as  fast  as  the  earth  (p.  205).  Its  light 
reaches  the  earth  in  twenty-five  years.  Mirach  (t)  lies 
in  the  girdle,  6  in  the  right  shoulder,  Alkaturops  (//) 
in  the  club,  /3  in  the  head,  and  Seginus  (y)  in  the  left 
shoulder.  Seginus  forms  with  Cor  Caroli  and  Arc- 
turus  a  triangle,  right-angled  at  Seginus.  Three 
small  stars  in  the  left  hand  of  Bootes  lie  near 
Benetnasch. 

Mythological  History. — Bootes  is  supposed  to  have  been  Areas,  the 
son  of  Callisto.     (See  Ursa  Major.) 

Hercules  is  represented  as  a  warrior  clad  in  the 
skin  of  the  Nemsean  lion,  holding  a  club  in  his  right 
hand  and  the  dog  Cerberus  in  his  left.  His  foot  is 
near  the  head  of  Draco,  while  his  head  lies  38°  south 
and  his  club  reaches  10  degrees  beyond. 

The  Principal  Star  is  Ras  Algethi  (a  Herculis). 
This  forms  a  triangle  with  j3  and  S.  A  peculiar 
figure  of  four  stars  (tt,  ?/,  ^,  e),  north  of  these,  marks 
the  body.  (See  Maps,  Nos.  5,  6,  and  7.)  The  left 
knee  is  pointed  out  by  0,  and  the  left  foot  by  y. 

Mythological  History. — This  constellation  immortalizes  the  name  of 
one  of  tlie  greatest  heroes  of  antiquity.  Hercules  was  the  son  of  Jupiter 
and  Alcmena.  "While  he  was  yet  lying  in  his  cradle,  Juno,  in  her  jealousy, 
sent  two  serpents  to  destroy  him.  The  precocious  infant,  however, 
strangled  them  with  his  hands.  By  the  cunning  artifice  of  Juno,  Hercules 
was  made  subject  to  Eurystheus,  his  elder  half-brother,  and  compelled  to 
perform  all  his  commands.  Eurystheus  enjoined  upon  him  a  series  of  the 
most  difficult  and  dangerous  enterprises  that  could  be  conceived,  which  have 
been  termed  the  "  Twelve  Labors  of  Hercules. "  Having  completed  these 
tasks,  he  afterward  achieved  others  equally  celebrated.  Near  the  close  of 
his  life  he  killed  the  centaur  Nessus.  The  dying  monster  charged  Dejanira, 
the  wife  of  Hercules,  to  preserve  a  portion  of  his  blood  as  a  charm  to  use 


234  tHE   SIDEREAL  SYSTEM. 

ill  case  the  love  of  her  husband  should  ever  fail  her.  In  time,  Dejanira 
thought  she  needed  the  potion,  and  Hercules  having  sent  for  a  white  robe 
to  wear  at  a  sacrifice,  she  steeped  the  garment  in  the  blood  of  Xessus.  Xo 
sooner  had  Hercules  put  on  the  fatal  robe  than  the  venom  stung  his  bones 
and  boiled  through  his  veins.  He  attempted  to  tear  it  off,  but  in  vain. 
It  stuck  to  his  flesh,  and  tore  off"  great  pieces  of  his  bodj\  The  hero, 
finding  he  must  die,  ascended  Mount  (Eta,  where  he  erected  a  funeral  pyre, 
spread  out  the  skin  of  the  Xemaean  lion,  and  laid  himself  down  uiion  it. 
PhUoctetes  applied  the  torch.  With  perfect  serenity  of  countenance 
Hercules  awaited  approaching  death — 

"  Till  the  gofl,  the  earthly  part  forsaken, 

From  the  man  in  flames  asunder  taken. 

Drank  the  heavenly  ether's  purer  breath. 

Joyous  in  the  new  unwonted  lightness 

Soared  he  upward  to  celestial  brightness, 

Earth's  dark,  hea\-y  burden  lost  in  death." — Schiller. 

Corona  consists  of  six  stars  arranged  in  a  semi- 
circular form.  The  brightest  of  these  is  Alphecca. 
This  makes  a  triangle,  with  Mirach  (^)  and  (J  in  Bootes. 
It  forms  a  similar  figure  with  Mirach  and  Arcturus. 

SerjjentariiiSf  or  Ophiuch  tis^  the  serpent-bearer, 
is  represented  under  the  figure  of  a  man  grasping  in 
both  hands  a  prodigious  serpent,  which  is  writhing 
in  his  grasp. 

Principal  Stars. — Ras  Alhague  {a),  in  the  head,  is 
of  the  second  magnitude.  It  is  about  5°  from  Ras 
Algethi.  They  form  a  pair  of  stars  conspicuous  like 
the  pairs  in  Gemini,  Canis  Minor,  Canis  Major,  etc. ; 
3  marks  the  right  shoulder,  and  k  the  left.  There  is 
a  small  cluster  near  3,  called  Taurus  Poniatowskii. 
An  irregular  square  of  four  stars,  near  y  Herculis, 
denotes  the  head  of  the  serpent. 

Mythological  History. — This  constellation  perpetuates  the  memory 
of  JEsculapius,  the  father  of  medicine.  He  was  so  skilful  that  he  restored 
several  persons  to  life  ;  whereupon   Pluto  complained  to  Jupiter  that  his 


fiQUATOHlAL  CONSTELLATIONS. 


235 


kingdom  was  in  clanger  of  being  depopnlateil.  Therefore  Jupiter  struck 
him  with  a  tliunderbolt,  but  afterward  placed  him  among  the  constel- 
lations. Serpents  were  sacred  to  ^sculapius,  because  of  the  superstitious  idea 
that  they  have  the  power  of  renewing  their  youth  by  changing  their  skin. 

Libra  represents  the  scales  of  Astraea  (Virgo),  the 
goddess  of  justice.     It  may  be  recognized  by  the 

(Map  No.  G)—Fig.  92. 


quadrilateral  figure  formed  by  its  four  principal 
stars. 

Scorpio  is  represented  under  the  figure  of  a  huge 
scorpion,  stretching  through  25°.  It  is  a  most  in- 
teresting constellation. 

Principal  Stars. — Antares  («)  is  a  fiery  red  star  of 
the  first  magnitude.  It  marks  the  heart  of  the  Scor- 
pion.    The  head  is  indicated  by  several  stars,  the 


^36  THE  siDeHeal  system. 

most  prominent  of  which  is  §,  arranged  in  a  line 
slightly  curved.  The  tail  may  be  easily  traced  by  a 
series  of  stars  which  winds  around  through  the  Milky 
Way  in  a  beautiful  manner.  * 

Mythological  Hlstory. — This  is  the  scorpion  that  sprang  out  of  the 
earth  at  the  command  of  Juno,  and  stung  Orion.  Scorpio  and  Orion  are 
so  placed  among  the  constellations  that  they  never  appear  in  the  heavens 
together. 

Sagittarius^  the  archer,  is  represented  as  a  cen- 
taur with  his  bow  bent,  as  if  about  to  let  fly  an  arrow 
at  Scorpio. 

Principal  Stars. — A  row  of  stars  from  ju  to  /?  marks 
the  bow :  another  from  }-  eastward  points  out  the 
arrow  and  the  right  arm  drawn  back  in  bending  the 
bow.  North  of  r,  two  stars  of  the  fourth  magnitude 
denote  the  head  of  the  centaur.  The  3Iilk  Dipper, 
so  called  because  the  handle  lies  in  the  Milky  Way, 
is  a  very  striking  figure. 

Mythological  History. — This  constellation  is  named  in  honor  of 
Chiron,  one  of  the  centaurs.  These  monsters  were  represented  as  men  from 
the  head  to  the  loins,  while  the  remainder  of  the  body  was  that  of  a  horse 
— the  ancients  having  so  high  an  opinion  of  that  animal  that  the  union 
was  not  considered  in  the  least  degrading. 

Chiron  was  renowned  for  his  skill  in  music,  medicine,  and  prophecy. 
The  most  distinguished  heroes  of  mythology  were  among  his  pupils.  He 
taught  j^sculapius  phj'sic  ;  Apollo,  music  ;  and  Hercules,  astronomy. 

Cajyricoriias  contains  no  very  conspicuous  stars. 
The  Southern  Fish  (No.  6)  has  one  star  of  the  first 
magnitude,  Fomalhaut  («,  No.  7),  which  on  a  clear 
summer  evening  may  be  seen  in  the  south,  midway 
to  the  zenith.     Antinous  and  the  Eagle  is  a  double 

*  Antares  (anti,  like  ;  Ares,  Mars)  was  so  named  because  it  rivalled  Mars  in  brightness 
and  color. 


EQUATORIAL  CONSTELLATIONS. 


m 


Constellation.  It  contains  a  beautiful  star  of  the 
first  magnitude,  Altair.  This  is  conspicuous,  as  be- 
ing the  center  one  in  the  row  of  three  bright  stars. 
A  similar  row  denotes  the  tail  of  the  eagle ;  the  first 
star  of  which  is  named  C,  and  the  last  star  lies  in 
Cerberus.  The  Dolphin  contains  a  pretty  cluster  in 
the  form  of  a  diamond.  It  is  sometimes  called  Job's 
Coffin. 

(Map  No.  7)-Fig.  93. 


Pe"g  ASU  S' 

.-... 

.  y 

Ta    ; 

/' 

c 

^EADVot 

';      '     -. 

s  ^ 

K-N- 

••'s 

«>  R  a'c  0 

~«'<1 

■\1 

■  ye 

-•    ' 

,'r 

■         V      . 

:     F    6 

X  . 

'fi 

// 

^" 

■    1    W' 

"^ 

-,    ,. 

^ 

ABROW 

/'  »J* 

•  • 

/• 

DOLPHIN' • 

\, 

/'    to  : 

r-Y-     ■■ 

. 

'     0    i 

^(a 

".'  • 

*f>' 

waterbe'arer 

S   A    G 

.  *   •'   .■■ 

• 
c 

.   •'a 

ANT  I'N  0  U   S    . 

'.*::• 

a 

s 

CygnuSf  the  swan,  is  a  remarkable  group  of  stars. 
the  principal  ones  being  so  arranged  as  to  form  a 
large  and  beautiful  cross.  The  upright  piece  lies 
along  the  Milky  Way.  It  is  composed  of  four  stars, 
three  of  which,  Deneb  (a),  y,  and  S,  are  bright,  while 
the  fourth  is  a  variable  star.  No.  61,  a  minute  star, 
scarcely  visible  to  the  naked  eye,  is  noted  as  being 
the  nearest  to  the  earth  of  any  of  the  fixed  stars  in 
the  northern  hemisphere  (p.  241). 


238  THE  SIDEREAL  SYSTEM. 

LyrOf  the  harp,  contains  one  brilliant  blue  star, 
Vega  (p.  217).  Close  by  it  is  a  parallelogram  of  four 
smaller  stars,  by  which  it  may  be  easily  recognized. 

.Mythological  History.— This  is  the  celestial  lyre  upon  which  Orph- 
eus discoursed  such  ravishing  music  that  Avild  beasts  forgot  their  fierceness 
and  gathered  about  him  to  listen,  while  the  rivers  ceased  to  flow,  and  the 
very  rocks  and  trees  stood  entranced. 

III.  Southern  Constellations.— AVe  now  imagine 
ourselves  viewing  the  stars  visible  to  a  person  far 

(Map  No.  8)— Fig.  91,. 


SOUTHERN 
■--.-HYDRUS  .  .■..;;.       CROSS 


C. 


-?> 


1^    O    -R    K^  ' 


^Jp/ak'6V 


^^eat  DOG* 


south  of  the  equator.  The  constellations  are  re- 
versed with  reference  to  the  horizon.  The  two  stars 
which,  in  the  northern  hemisphere,  compose  the  base 
of  the  parallelogram  in  Orion,  form  here  the  upper 
side.  Sirius  is  above  Orion.  All  the  northern  cir- 
cumpolar  constellations  are  hidden  from  view.  At 
the  southern  pole  there  is  no  conspicuous  star,  but 
the  richness  and  number  of  the  neighboring  stars 
compensate  this  deficiency,  and  give  to  the  heavens 


THE   SOUTHERN   CONSTELLATIONS.  23d 

an  incomparable  splendor.  Here  is  the  magnificent 
constellation  Argo,  in  which  we  find  Canopus,  looked 
upon  anciently  as  next  to  Sirius  in  brilliancy  :  rj,  a 
variable  star,  now  surpasses  it  in  brightness. 

Nearly  at  the  height  of  the  south  pole,  blazes  the 
Southern  Cross;  below  is  the  Centaur,  containing 
two  stars  of  the  first  magnitude  and  five  of  the 
second  ;  and  above  is  Hydrus,  where  shines  Acher- 
nar,  another  beautiful  star  of  the  first  magnitude. 


III.   DOUBLE  STARS,  COLORED 
STARS,  NEBUL^^,  ETC. 

1.  Double  Stars.— To  the  naked  eye,  all  the  stars 
appear  single.  With  the  telescope,  over  10,000  have 
been  found  to  be  double.  Thus,  Polaris  consists  of 
two  stars  about  18"  apart ;  Rigel  has  a  companion 
about   10"  from  it ;   and  Sirius,   one  distant   7".     A 


240  THiJ  SIDEflEAL  SYSTEM. 

good  opera-glass  will  separate  £  Lyrse  into  two 
components. 

In  case  two  stars  happen  to  lie  in  the  same  straight 
line  from  us,  though  at  immense  distances  from  each 
other,  their  light  will  blend.  They  will  be  seen  by 
the  naked  eye  as  a  single  star,  and  by  the  telescope 
as  a  double  star.  They  are  called  optical  double 
stars.  Many,  however,  of  the  double  stars  have  been 
found  to  be  physically  connected.  Each  double  star 
of  this  class  forms  a  binary  system  of  two  suns  re- 
volving in  an  elliptical  orbit  about  their  common 
center  of  gravity,  like  the  planets  in  the  solai*  system, 
in  accordance  with  Newton's  law  of  gravitation.  In 
a  few  instances,  there  are  combinations  of  triple, 
quadruple,  and  even  septuple  stars.  Thus  e  Lyrae  is 
a  double-double  star,  while  0  Orionis  is  a  system  of 
six  suns.  The  components  of  a  double  star  com- 
monly differ  in  brightness ;  so  that  frequently  the 
fainter  one  is  nearly  lost  in  the  brilliancy  of  its  com- 
panion sun. 

The  Periods  of  some  systems  have  been  ascer- 
tained. Thus,  I  Ursae  Majoris  is  a  double  star,  and 
the  two  stars  of  which  it  is  composed  have  performed 
an  entire  revolution  about  each  other  since  they 
were  found  to  be  connected.  There  are  only  eleven 
binary  stars  now  known  whose  periods  are  less 
than  a  century,  while  the  others  have  periods  which 
seem  to  extend,  in  some  cases,  beyond  a  thousand 
years. 

Orbits. — It  is  not  possible  to  estimate  the  dimen- 
sions of  the  orbits  of  the  double  stars,  until  their  dis- 
tances from  us  are  definitely  known.     ''Taking  the 


DOUBLE  STARS.  241 

estimated  distance  of  61  Cygni  (550,000  times  the 
sun's  distance  from  the  earth)*  as  a  basis,  the  com- 
panions of  that  system  cannot  cultivate  a  very 
intimate  acquaintance,  since  they  must  be  over  a 
billion  miles  apart.  From  these  data,  astronomers 
have  attempted  even  to  calculate  the  mass  of  some 
of  the  double  stars.  61  Cygni,  although  scarcely 
visible  to  the  naked  eye,  and  known  to  be  the  sec- 
ond nearest  to  us  of  any  of  the  fixed  stars,  is  esti- 
mated to  weigh  one-third  as  much  as  our  sun."  (See 
p.  308.) 

II.  Colored  Stars. — We  Kave  already  noticed  that 
the  stars  are  of  various  colors. f  Sirius  is  white; 
Antares,  red;  and  Capella,  yellow ;  while  Lyra  has  a 
blue  tint,  and  Castor  has  a  green  one.  In  the  pure 
transparent  atmosphere  of  tropical  regions,  the 
colors  are  far  more  brilliant.  There,  oftentimes, 
the  nocturnal  sky  is  a  blaze  of  jewels, — the  stars 
glittering  with  the  green  of  the  emerald,  the  blue 
of  the  amethyst,  and  the  red  of  the  topaz. 

In  the  double  and  multiple  stars,  every  color  is 
presented  in  all  its  richness  and  beauty  ;  while  there 
are  also  combinations  of  colors  complementary  to 
each  other.  Here  is  a  green  star  with  a  blood-red 
companion  ;  here  an  orange  and  a  blue  sun  ;  there  a 
yellow  and  a  purple  one.  The  triple  star  y  Andro- 
medse  is  formed  of  an  orange-red  sun  and  two  others 
of  an  emerald  green. 

Every  tint  that  blooms  in  the  flowers  of  summer, 

♦  Recent  measurements  of  this  star  seem  to  indicate  its  probable  distance  from  the 
Bun  to  be  400,000  radii  of  the  earth's  orbit. 

t  The  theory  has  been  advanced  that  the  color  indicates  the  intensity  of  the  heat  oj 
(he  st?r.    A  white  star  is  tlierefore  hotter  than  a  red  star  ;  and  a  blue,  than  a  jellow  one 


243  THE  SIDEREAL  SYSTEM, 

flames  out  in  the  stars  at  night.  "The  rainbow 
flowers  of  the  footstool  and  the  starry  flowers  of  the 
throne,"  proclaim  their  common  Author  ;  while  rain- 
bow, flower,  and  star  alike  evince  the  same  Divine 
love  of  the  beautiful. 

We  can  hardly  conceive  the  effects  produced  in  a 
system  having  colored  suns.  Take  a  planet  revolv- 
ing about  tp  Cassiopeise  for  instance.  This  is  illu- 
minated by  a  red,  a  blue,  and  a  green  sun.  Some- 
times, by  the  succession  of  these  suns,  a  cheerful 
green  day  would  present  a  charming  relief  to  a  fiery 
red  one ;  and  that  might  be  still  further  subdued  by 
a  gentle  blue  one.  The  odd  contrast  of  color  and  the 
vicissitudes  of  extreme  heat  and  cold  that  obtain  on 
such  a  world,  present  a  picture  which  our  fancy  can 
sketch  better  than  words  can  paint. 

The  colors  of  the  stars  change.  Sirius  was  ancient- 
ly red.  It  is  now  unmistakably  white.  There  are 
two  double  stars  which  were  described  by  Herschel 
as  white  ;  each  is  now  composed  of  a  golden-yellow 
and  a  greenish  star. 

III.  The  Variable  Stars  have  periodic  changes  of 
brilliancy.    The  following  are  most  conspicuous  : 

Algol,  in  the  head  of  Medusa,  is  a  star  of  the 
second  magnitude  for  about  two  and  a  half  days, 
when  it  suddenly  decreases,  and  in  three-and-a-half 
hours  descends  to  the  fourth  magnitude.  It  then  re- 
kindles, and  in  three-and-a-half  hours  is  again  as 
brilliant  as  ever. 

MiRA,  the  wonderful,  a  star  in  the  Whale,  has  a 
period  of  eleven  months.  It  is  ordinarily  of  the 
second  magnitude  for  about  fifteen  days.    It  then 


DOUBLE  STARS.  243 

decreases  for  three  months,  until  it  becomes  invisible 
to  the  naked  eye.  This  period  of  darkness  lasts  five 
months  ;  it  then  rebrightens  for  three  months,  until 
it  regains  its  former  lustre.  Occasionally,  however, 
it  fails  to  brighten  at  all  beyond  the  fourth  magni- 
tude, while  on  one  occasion  its  light  was  almost  equal 
to  that  of  Aldebaran.  Sometimes  no  perceptible 
change  takes  place  for  a  month  ;  then  again,  there  is 
a  sensible  alteration  in  a  few  days. 

The  Reason  of  this  Variability  is  not  under- 
stood. It  has  been  suggested,  in  the  case  of  Mira, 
that  it  may  be  a  globe  rotating  on  its  axis,  and  that 
different  portions  of  its  surface,  illuminated  to  differ- 
ent degrees  of  intensity,  are  thus  presented  to  us. 
Others  have  conceived  that  there  may  be  satellites 
revolving  about  these  suns,  and  that  when  their  dark 
bodies  interpose  between  the  stars  and  our  earth, 
they  eclipse  the  light  wholly  or  in  part. 

IV.  The  Temporary  Stars  suddenly  blaze  out  in 
the  heavens,  and  then  gradually  fade  away.  The 
most  celebrated  one  burst  forth  in  Cassiopeia,  in  the 
year  1572.  Tycho  Brahe  says  :  "  One  night  as  I  was 
examining  the  celestial  vault,  I  saw  with  unspeak- 
able astonishment  a  star  of  extraordinary  brightness 
in  Cassiopeia.  Struck  with  surprise,  I  could  scarcely 
believe  my  eyes.  To  convince  myself  that  there  was 
no  illusion,  1  called  the  workmen  of  my  laboratory 
and  the  passers-by,  and  asked  them  if  they  saw  the 
star  which  had  so  suddenly  made  its  appearance. 
It  could  be  compared  only  with  Venus  at  her  quad- 
rature, being  seen  distinctly  at  midday."  Its  color 
was  at  first  white,  then  yellow,  and  finally  red.    Its 


244  THE  SIDEREAL  SYSTEM. 

brightness  decreased  gradually  until  the  spring  of 
1574,  when  the  star  disappeared  from  view  and  has 
not  since  been  seen.  As  two  brilliant  stars  had  pre- 
viously appeared  in  Cassiopeia,  at  intervals  of  about 
three  centuries,  they  have  been  thought,  by  some,  to 
be  identical,  and  that  it  is  only  a  variable  star  of  long 
period. 

Since  this  discovery  by  Tycho  Brahe,  numerous  in- 
stances are  recorded  of  stars  which  have  suddenly 
burst  forth,  and  have  then  either  faded  out  entirely, 
or  remain  as  faint  telescopic  objects.  In  the  latter 
case,  they  are  termed  New  stars.  One  of  this  kind 
appeared  in  Corona  Borealis,  in  1866.  At  first  it  was 
of  the  second  magnitude,  but  in  a  week  changed  to 
the  fourth,  and  in  a  month  diminished  to  the  ninth. 
Strangely,  too,  some  stars  have  disappeared  from  the 
heavens,  and  are  styled  Lost  stars.  The  changes 
which  are  thus  constantly  taking  place  are  calculated 
to  make  the  term  *' eternal  stars"  seem  a  very  in- 
definite phrase. 

Explanation. — ^These  phenomena  are  as  yet  little 
understood.  A  rotation  about  an  axis  would  fail 
to  explain  the  changes  in  color.  Some  think  that 
these  stars  revolve  in  enormous  orbits  of  such  eccen- 
tricity that  at  their  most  distant  points  they  fade  out 
of  sight.  Arago  has  shown,  in  reply  to  this,  that  for 
a  star  to  decrease  in  brightness  from  the  first  magni- 
tude to  the  second  by  moving  directly  from  us,  even 
with  the  velocity  of  light,  would  require  six  years. 
As  we  have  just  seen,  the  star  of  1866  underwent  this 
change  in  brilliancy  in  a  week. 

The  mind  cannot  help  wondering  if  they  are  oot 


DOUBLE  STARS.  245 

instances  of  enormous  conflagrations  in  which  a 
world  is  overwhelmed  in  ruin  !  The  investigations 
of  spectrum  analysis  indicate  that  the  star  of  1866 
consisted  of  burning  hydrogen  gas.  We  can  suppose 
that  the  gas  was  evolved  by  some  convulsion,  and, 
taking  fire,  wrapped  the  entire  globe  in  flames. 
This  need  not  involve  the  idea  of  destruction,  but 
only  a  change  of  form.  A  dark  star  may  thus 
become  luminous,  or  a  bright  one  may  be  extin- 
guished.* 

5.  The  Star  Clusters  are  groups  of  stars  so  massed 
together  as  to  present  a  hazy,  cloud-like  appearance. 
Several  of  them  have  been  already  named, — the  Plei- 
ades, the  Beehive  in  Cancer,  Berenice's  Hair,  the 
Hyades,  and  the  group  in  the  sword-handle  of  Per- 
seus. The  principal  stars  of  which  they  are  com- 
posed can  generally  be  distinguished  by  the  naked 
eye,  although  by  the  use  of  a  small  opera  or  spy- 
glass the  number  is  increased. 

In  the  southern  sky,  there  are  clusters  still  more 
remarkable.  In  the  Cross,  is  a  group  of  110  stars  of 
various  colors,  red,  blue,  and  green,  so  that  looking 
on  it,  says  Herschel,  is  "like  gazing  into  a  casket  of 
precious  gems."  A  cluster  in  Toucan  is  compact  in 
the  center,  where  it  is  of  an  orange-red  color ;  the 
exterior  is  composed  of  pure  white  stars,  making  a 
border  of  exquisite  contrast. 

It  is  generally  conceded  that  there  is  some  close 

*  The  x>rocess  of  apparent  creation  and  destruction  is  going  on  in  the  heavens  im- 
mediately before  the  eye  of  the  astronomer.  New  stars  flash  light,  old  stars  are  lost, 
worlds  burst  into  flame,  and  their  glowing  embers  fade  into  darkness.  Are  they  re- 
created into  new  worlds?  We  know  not.  We  only  perceive  that  the  same  Almighty 
power  which  fitted  up  this  earth  for  our  home  is  yet  at  work  among  the  worlds  about 
us,  and  we  are  thus  witnesses  of  His  eternal  presence. 


246 


THE  SIDEREAL    SYSTEM. 

Fig.  9e. 


Star-duster  in  Toucan. 

physical  relation  existing  between  the  stars  compos- 
ing such  an  ''archipelago  of  worlds,"  but  its  nature 
is  a  mystery.  They  seem  generally  crowded  together 
toward  the  center,  blending  into  a  continuous  blaze 
of  light.  Yet,  although  they  appear  so  densely  com- 
pacted, it  is  probable  that,  if  we  could  change  our 
stand-point  and  penetrate  one  of  these  gi'oups  *of 
suns,  we  should  find  it,  on  our  approach,  opening  up 
and  spreading  out  before  us,  until,  in  the  midst,  the 
suns  would  shine  down  upon  us  from  the  heavens  as 
the  stars  do  in  our  own  sky. 

6.  Nebulae  are  faint,  misty  objects,  like  specks  of 
luminous  clouds.  A  few  are  visible  to  the  naked  eye, 
but  the  telescope  reveals  thousands.  They  differ 
from  clusters  in  not  being  resolvable  into  stars  when 
viewed  through  the  largest  telescopes.   With  the  con- 


NEBUL.^.  247 

stant  improvement  made  in  these  instruments,  how- 
ever, many  so-called  nebulae  have  been  resolved,  and 
thus  the  number  of  clusters  has  been  increased, 
while  new  nebulae  have  been  discovered. 

Until  of  late,  it  was  thought  that  all  nebulae  were 
simply  groups  of  stars,  which  would  be  ultimately 
discerned  in  the  more  powerful  telescopes  yet  to  be 
made.  Spectrum  analysis  shows,  however,  that 
many  of  these  luminous  clouds  are  gaseous,  and  are 
not  composed  of  stars. 

Since  all  the  nebulae  maintain  the  same  position 
with  respect  to  the  stars,  their  distance  must  be 
inconceivably  great,  and,  in  order  to  be  visible  to  us, 
their  magnitude  must  be  proportionately  vast.  They 
are  most  abundant  at  the  two  poles  of  the  Milky 
Way,  but  are  more  uniformly  distributed  over  the 
heavens  lying  near  the  south  pole. 

It  is  now  generally  believed  that  nebulae  constitute 
the  material  for  making  stars, — are,  in  fact,  sun- 
germs  ;  that  all  stars  originally  existed  as  nebulae  ; 
and  that  every  nebula  will,  in  time,  be  changed  into 
stars. 

Nebulae  are  divided,  according  to  their  form,  into 
six  classes — elliptic,  annular',  spiral, planetary ,  irreg- 
ular nebulce,  and  nebulous  stars,  f 

The  Elliptic  or  merely  oval  nebulae  are  the  most 
abundant.  Under  this  head  is  classed  the  Great 
Nebula  in  Andromeda,  which  was  discovered  over 

t  This  division  of  the  nebulae  is  purely  arbitrary,  and  used  only  to  introduce  some 
order  of  arrangement.  The  shape  of  the  nebulae  changes  with  the  power  of  the  telescope 
through  which  they  are  seen.  Thus  the  Great  Nebula  in  Andromeda,  as  resolved  by 
Bond,  is  no  longer  oval,  but  irregular  in  form.  The  Ring- Nebula  of  Lyra,  seen  through 
the  large  telescope  of  to-day,  is  egg-shaped;  while  the  Dumb-bell  Nebula  assumes  the 
outline  of  a  chemical  retort 


248 


THE  SIDEREAL  SYSTEM. 


a  thousand  years  ago,  and  is  visible  to  the  naked 
eye.     Prof.  Bond,   of  the  Cambridge  Observatory, 
jr,g  97  has  partly  resolved  it  into  stars, 

of  which  he  has  counted  1500, 
although  its  nebulous  appear- 
ance was  still  retained.  Through 
the  telescope,  it  is  one  of  the 
most  glorious  objects  in  the 
heavens.  "If  we  suppose  this 
nebula  to  be  one  continuous  bed 
of  stars  of  different  sizes  for  its 
entire  extent,  it  must  comprise 
the  enormous  number  of  30,000,000." 

The  distance  of  such  nebulse  from  the  earth  passes 
our  comprehension.  Some  astronomers  have  esti- 
mated that  a  ray  of  light  would  require  800,000  years 
to  span  the  gulf  that  intervenes.  Imagination  wearies 
itself  in  the  attempt  to  understand  these  figures. 
They  teach  us  something  of  the  limitless  expanse  of 
that  space  in  which  God  is  working  the  mysterious 
problem  of  creation. 

Fid.  9S. 


Nebula  in  Andromeda. 


ycbula  in  Lyra. 


The  Annular  2s  ebulje  have  the  form  of  a  ring. 
There  are  four  of  these  **ring  universes."    In  the  cut 


NEBULA. 


249 


is  a  representation  of  one  in  Lyra, — first,  as  seen  by 
Herscliel,  having  in  the  center  a  nebulous  film  like  a 

Fig.  99. 


Spiral  Cluster  in  Cane*  Venatki. 


250 


THE  SIDEREAL  SYSTEM. 


"bit  of  gauze  stretched  over  a  hoop;"  second,  as 
sho-wn  in  Lord  Rosse's  telescope  (p.  II),  which  re- 
solves the  filmy  parts  of  the  nebula  into  minute  stars, 
and  reveals  a  fringe  of  stars  along  the  edge. 

The  Spiral  or  Whirlpool  Xebul^  are  exceedingly 
curious.  The  most  remarkable  one  is  in  Canes  Vena- 
tici.  It  consists  of  brilliant  spirals  sweeping  outward 
from  a  central  nucleus,  and  all  overspread  with  a 
multitude  of  stars.*  One  is  lost  in  attempting  to 
imagine  the  distance  of  such  a  mass,  and  the  forces 
which  produce  such  a  "tremendous  hurricane  of 
matter — perhaps  of  suns." 

Planetary  Nebula,  by  their  circular  form  and 
pale,  uniform  light,  resemble  the  disks  of  the  distant 
planets  of  our  system.  Their 
edges  are  generally  well  de- 
fined, though  sometimes  slightly 
furred.  There  is  one  in  Ursa 
Major,  which,  if  located  at  the 
distance  of  61  Cygni,  would 
"fill  a  space  equal  to  seven 
times  the  entire  orbit  of  Nep- 
tune." 

Planetary  Nebula  IRREGULAR   NEBUL^  are  thoSO 

which  have  no  definite  form.  Many  present  the 
irregularities  of  clouds  torn  by  the  tempest.  Some 
of  the  likenesses  which  may  be  traced  are  strangely 
fantastic  :  for  example,  the  Dumb-bell  Xebula,  in  the 
constellation  Vulpecula,  and  the  Crab  Xebula,  near 
the  southern  horn  of  Taurus.  There  is  also  one  known 


Fig.  100. 


*  Columbus  discovered  a  new  continent,  and  so  immortalized  his  name  ;  what  shall 
we  say  of  the  astronomer  who  discovers  a  system  of  worlds  ? 


NEBULA. 


261 


as  the  Great  Nebula  ""''■  '"'■ 

in  the  Sword-handle 
of  Orion,  which  bears 
a  faint  resemblance 
to  the  wings  of  a  bird. 
Nebulous  Stars 
are  so  called  because 
they  are  enveloped 
by  a  faint  nebula, 
usually  of  a  circular 
form.  The  star  is 
generally  seen  at  the 
center,  although 
some  nebulae  sur- 
round   two    stars,  Dumb-bell  Nebula. 

having  one  in  each  focus.  It  is  thought  that  these 
may  be  suns  possessing  immense  atmospheres,  which 
are  rendered  visible  somewhat  as  that  of  our  sun  is  in 
the  zodiacal  light ;  and  that  in  like  manner  our  sun 
may  present  to  other  worlds  the  appearance  of  a 
nebulous  star.* 
Variable  Nebulje. — Certain  changes  take  place 


♦  Nothing  in  all  nature  is  more  suggestive  of  the  magnificence  and  immensity  of 
Creation,  than  are  the  nebulous  star-clusters,  many  of  which  are  at  such  an  inconceiv-. 
able  distance,  that  the  most  powerful  telescopes  show  them  only  as  a  confused  mass  of 
light.  A  casual  observer, — even  though  when  led  by  scientific  analogy  to  resolve  each 
little  patch  of  star-dust  into  a  host  of  separate  suns,  and  to  provide  each  sun  with  a 
retinue  of  inhabited  planets,— might  think  of  them  as  little  colonies  of  suns,  set  on  the 
very  outskirts  of  world-creation,  and  moving  in  such  close  proximity,  that  the  peoples 
of  the  various  worlds  might  communicate  with  one  another.  Yet,  were  he  transported 
to  some  planet  whirling  about  one  of  those  far-otf  star-suns,— a  multitude  of  which 
blend  as  a  single  point  of  light  to  our  human  eyes,— he  would  see  the  other  suns  only  as 
fixed  stars  in  the  firmament  above  him ;  and  though  many  of  them  might  surpass  in 
splendor  the  glory  of  our  own  Sirius,  yet  all  would  still  remain  at  such  an  immense 
distance  as  to  baffle  the  research  of  the  most  powerful  telescopic  instruments.  Thus,  too, 
he  would  probably  find  each  planet  revolving  at  such  a  distance  from  its  sister  planets, 
as  to  render  the  certain  knowledge  of  other  inhabited  worlds  as  elusive  there  as  here. 


952 


THE  SIDEREAL  SYSTEM. 

Fig.  IVS. 


Crab  Nebula, 

among  the  nebulae  which  can  be  accounted  for  only 
under  the  supposition  that  they,  like  some  of  the 
stars,  are  variable.  Mr.  Hind  tells  us  of  a  nebula  in 
Taurus  which,  in  1852,  was  distinctly  visible  with  a 
small  telescope,  but,  in  1862,  had  vanished  entirely  out 
of  the  reach  of  a  powerful  instrument.  The  Great 
Nebula  in  Argo,  when  observed  by  Herschel  in  1838, 


had  in  the  center  a  vacant  space  containing  a  star  ot 
the  first  magnitude,  enshrouded  by  nebulous  matter. 
In  1863,  the  nebulous  matter  had  disappeared,  and 
the  star  was  only  of  the  sixth  magnitude.  These 
facts  as  yet  defy  explanation.  They  illustrate  the 
vast  and  wonderful  changes  constantly  taking  place 
in  the  heavens. 

Double  Nebula. — There  seems  to  be  a  physical 
connection  existing  between  some  of  the  nebulae, 
similar  to  that  already  noticed  in  respect  to  certain 
stars.  In  the  case  of  the  latter,  this  inter-relation 
has  been  proved,  since,  even  at  their  distances,  their 
movements  can  yet  be  traced  in  the  lapse  of  years. 
'*  But,  owing  to  the  almost  infinite  depths  in  the  abyss 
of  the  heavens  at  which  these  nebulae  exist,  thou- 
sands of  years,  perhaps  thousands  of  centuries, 
would  be  necessary  to  reveal  any  movement." — 
(Guillemin.) 

7,  Magellanic  Clouds. — ITot  far  from  the  southern 
pole  of  the  heavens,  there  are  two  cloud-like  masses, 
distinctly  visible  to  the  naked  eye,  known  to  navi- 
gators as  Cape  Clouds.  Sir  John  Herschel  describes 
them  as  consisting  of  swarms  of  stars,  clusters,  and 
nebulae,  seemingly  grouped  together  in  the  wildest 
confusion.  In  the  larger,  he  found  582  single  stars, 
46  clusters,  and  281  nebulae. 

8.  The  Milky  Way. — Via  Lactea,  or  the  Galaxy,  is 
a  luminous,  cloud-like  band  that  stretches  across  the 
heavens  in  a  great  circle.  It  is  inclined  to  the  celes- 
tial equator  about  63°.  This  stream  of  suns  is 
divided  into  two  branches  from  a  Centauri  to  Cyg- 
nus.     To  the  naked  eye,  it  presents  merely  a  diffused 


254  iHEl  SIDEREAL  SYSTEM. 

light ;  but  with  a  large  telescope  it  is  found  to  con- 
sist of  myriads  of  stars  densely  crowded  together.* 

These  stars  are  not  uniformly  distributed  through 
the  entire  extent.  In  some  regions,  within  the  space 
of  a  single  square  degree  we  can  discern  as  many  as 
can  be  seen  with  the  naked  eye  in  the  entire  heavens. 
In  other  parts,  there  are  broad,  open  spaces.  A 
remarkable  instance  of  this  occurs  nears  the  South- 
ern Cross.  There  is  a  dark,  pear-shaped  vacancy, 
with  a  single  bright  star  at  the  center,  glittering  on 
the  blue  background  of  the  sky.  In  viewing  it,  one 
is  said  to  be  impressed  with  the  idea  that  he  is  looking 
through  an  opening  into  the  starless  depths  beyond 
the  Milky  Way. 

The  northern  galactic  pole  is  situated  near  Coma 
Berenices,  and  the  southern  in  Cetus.  Advancing 
from  either  pole  toward  the  Milky  Way,  the  number 
of  stars  increases,  at  first  slowly  and  then  more 
rapidly,  until  the  proportion  at  the  galaxy  itself  is 
thirty-fold. 

Fig.  103. 


HerscheVa  Theory  of  the  Stellar  System, 

*  Herschel  states  that  258,000  stars  once  passed  across  tlie  field  of  his  great  reflector 
in  41  minutes.  With  the  powerful  instruments  now  making,  it  is  probable  that  many 
more  could  be  seen. 


Nebula.  S55 

Herschel's  Theory.* — Sir  W.  Herschel  has  con- 
jectured that  the  stars  are  not  indifferently  scattered 
through  space,  but  are  collected  in  a  stratum  some- 
thing like  that  shown  in  the  cut,  and  that  our  sun 
occupies  a  place  at  S,  near  where  the  stream 
branches,  A  and  E  being  the  galactic  poles.  It  is  evi- 
dent that,  to  an  eye  viewing  the  stratum  of  stars  in 
the  direction  SB,  SC,  or  SD,  they  would  seem  much 
denser  than  in  the  direction  SA  or  SE.  Thus  are  we 
to  think  of  our  own  sun  as  a  star  of  the  second  or 
third  magnitude,  and  of  our  little  solar  system  as 
plunged  far  into  the  midst  of  this  vortex  of  worlds,  a 
mere  atom  along  that 

"  Broad  and  ample  road 
Whose  dust  is  gold  and  ])avenient  stars." 

9.  The  Nebular  Hypothesis  is  a  theory  advanced 
by  Laplace,  to  show  how  the  solar  system  may  have 
been  formed,  f  As  since  modified,  its  outlines  are  as 
follows:  In  the  "beginning,"  all  the  matter  which 
now  composes  the  sun,  and  the  various  planets  with 
their  moons,  was  in  a  gaseous  and  highly  heated 
state.  It  filled  the  space  at  present  occupied  by  the 
system,  and  extended  far  beyond  the  orbit  of  Nep- 
tune. In  other  words,  the  solar  system  was  simply 
an  immense  nebula.     The  heat,  which  is  the  repel- 

*  other  theories  have  been  advanced  by  astronomers,  but  we  are  as  yet  ignorant  of  the 
real  structure  of  the  universe  outside  of  our  own  system. 

t  We  should  remember  that  this  theory  aims  to  tell  only  the  way  in  which  our  system 
was  developed.  The  parent  nebula  must  have  confciined  a  potential  energy  equal  to  all 
the  manifestations  of  force  since  made  in  tlie  entire  system.  Nothing  could  be  devel- 
oped from  a  mass  of  nebulous  matter  the  germs  of  which  had  not  been  put  in  it  origin- 
ally by  the  Creator.  The  analogies  of  nature  all  go  to  show  that  the  Creator's  plan  is, 
in  general,  not  to  produce  any  object  in  a  perfect  and  matured  state  ;  but  rather,  by  a 
gradual  growth,  to  unfold  its  full  form  and  functiou. 


256  THE  SIDEREAL  SYSTEM. 

lant  force,  overcame  the  attraction  of  gravitation. 
Gradually  the  mass  cooled  by  radiation.  As  centu- 
ries passed,  the  repellant  force  becoming  weaker,  the 
attractive  force  drew  the  matter  and  condensed  it 
toward  one  or  more  centers.  The  nebula  then  pre- 
sented the  appearance  of  a  nebulous  star — a  nucleus 
enveloped  by  a  gaseous  atmosphere. 

According  to  a  well-known  law  in  physics,  seen 
in  every -day  life,  wherever  matter  seeks  a  center — 
as  in  a  whirlpool,  in  a  whirlwind,  or  even  in  water 
poured  through  a  funnel — a  rotary  motion  was  estab- 
lished. As  the  rotary  motion  of  the  nebula  increased, 
the  centrifugal  force  finally  overcame  at  the  exte- 
rior the  attraction  of  gravitation.  A  ring  of  condensed 
vapor  was  then  left  behind.*  Centuries  elapsed,  and 
again,  under  the  same  conditions,  a  second  ring  was 
detached.  Thus,  one  by  one.  concentric  rings  were 
separated  from  the  parent  nebula,  all  revolving  in 
the  same  plane  and  in  the  same  direction.  These 
different  rings,  becoming  gradually  consolidated, 
formed  the  planets.  Generally,  however,  in  this 
process,  while  still  in  the  vg^porous  state  and  slowly 
condensing,  the  rings  themselves  detached  other 
rings  that  were  in  turn  consolidated  into  satellites. 

In  the  case  of  Saturn,  several  of  these  secondary 
rings  did  not  condense  into  globes,  but  still  remain 
as  rings  which  revolve  about  the  planet,  f    Mitchell 

*A  eonadenbie  modification  of  the  Xelnilar  Hypothesis  is  possible,  leaving  its 
geneal  ides,  hcfwever,  intact.  It  is  dow  generally  conceded  that  the  several  planets 
■w&t  xtot  "tfciowB  oS,"  bat  merely  detached  and  left  behind.  Proctor  thinks  that  the 
solar  system  is  tbe  resolt  at  mteteork  a^gregaiion  as  well  as  gaseous  condensation :  the 
f*««»**g  jM  tikdr  iahiMCiy  beong  so  large,  gathered  immense  quantities  of  meteoroids, 


t  b  tte  eaae  attke  miaor  plaaeta  and  Ote  rings  of  Saturn,  we  may  suppose  tfaat  the 


NEBULAR   HYPOTHESIS.  257 

naively  remarks,  "  Saturn's  rings  were  left  unfinished 
to  show  us  how  the  world  was  made."  The  ring 
which  formed  the  minor  planets  broke  up  into  small 
fragments,  none  large  enough  to  attract  the  rest  and 
thus  form  a  single  globe. 

The  central  mass  of  vapor  finally  condensed  itself 
into  the  sun,  which  remains  the  largest  member  of 
the  system.  According  to  this  theory,  the  sun  may 
yet  give  off  a  few  more  planets,  whose  orbits  will  not 
exceed  its  present  diameter.  After  a  time,  all  its 
heat  will  be  radiated  into  space,  its  fire  will  become 
extinct,  and  life  on  the  planets  will  cease.  We  know 
not  when  this  remote  event  may  occur.  We  can- 
not fathom  the  purpose  of  God  in  creating  and  main- 
taining this  system  of  worlds,  nor  can  we  foretell 
how  soon  it  may  complete  its  mission.  We  are  as- 
sured, however, 

*'  That  nothing  walks  with  aimless  feet, 
That  not  one  life  shall  be  destroyed, 
Or  cast  as  rubbish  to  the  void, 
When  God  hath  made  the  pile  complete." 

In  Memokiam. 


rings  were  composed  of  matter  uniformly  distributed  ;  while  in  the  case  of  the  rings 
that  consolidated  into  planets,  there  was  a  nucleus  that  attracted  the  rest  of  the  matter 
to  itself.  It  is  possible  that  the  rings  of  Saturn  may  yet  break  up  and  form  new  satel- 
lites for  that  planet.  Indeed,  some  hold  that  one  at  least  of  the  rings  has  thus  been 
resolved  into  small  meteorites.  These  may  be  attracted,  and  .so  picked  up,  one  by  one, 
in  succession  by  the  larger,  until  they  form  another  moon,  which  will  continue  to  re- 
volve about  the  planet  as  the  ring  does  now. — "  The  present  state  of  the  .solar  system  is 
a  living  picture  of  the  entire  history  of  a  single  planet.  From  the  sun's  fire-mist,  to 
ring-girt  Saturn  ;  from  Saturn,  to  storm-beaten  Jupiter  ;  from  Jupiter,  to  the  sunny 
summer-time  of  our  own  planet ;  from  Earth,  to  autumn-browned  Mars  ;  and  from 
Mars,  to  the  wintry  silence  and  desolation  of  the  dark  gulches  of  the  moon,— there  is  a 
series  of  stages  that  carries  the  thought  back  into  the  eternity  long  passed,  as  well  as 
onward  into  the  measureless  depths  of  the  future,  and  confers  upon  human  intelligence 
a  sort  of  exemption  from  the  limitations  of  finite  existence."— Pro/.  Winchell. 


258  THE  SIDEREAL  SYSTEM. 


IV.    CELESTIAL    CHEMISTRY. 

Spectrum  Analysis. — The  rainbow — that  child  of 
the  sun  and  shower — is  familiar  to  all.  The  brilliant 
band  of  colors,  seen  when  the  sunbeam  is  passed 
through  a  prism,  is  scarcely  less  beautiful.  The  ray 
of  light  containing  the  primary  colors  is  here  spread 
out  fan-like,  and  each  tint  reveals  itself.  This  var- 
iously-colored band  is  called  a  spectrum  (plural, 
spectra).   There  are  three  different  kinds  of  spectra — 

1st.  When  the  light  of  a  solid  or  liquid  bod} ,  as 
iron  white-hot,  is  passed  through  a  prism,  the  spec- 
trum is  continuous,  and  consists  of  a  series  of  distinct 
colors,  varying  from  red  on  one  side  to  violet  on  the 
other. 

2nd.  If  the  light  of  a  burning  gas  containing  any 
volatilized  substance  be  passed  through  a  prism,  the 
spectrum  is  not  continuous,  but  is  ornamented  by 
bright-colored  lines, — sodium  giving  two  yellow  lines ; 
strontium,  a  red  one ;  silver,  two  beautiful  green  ones. 
Each  element  produces  a  definite  series  which  can 
be  recognized  as  its  test. 

3rd.  If  a  light  of  the  first  kind  be  passed  through 
one  of  the  second,  the  spectrum  is  crossed  by  dark 
lines.  Thus,  if  the  white  light  of  an  electric  lamp 
be  passed  through  a  flame  containing  sodium,  instead 
of  the  vivid  yellow  lines  so  characteristic  of  that 
metal,  two  black  lines  exactly  occupy  their  place. 
A  gaseous  flame  absorbs  the  rays  of  the  same  color  that 
it  emits.     (See  note,  \>.  310.) 


9 

3 


O    T3 


oo 


1      n 


CELESTIAL  CHEMISTRY. 


259 


The  Spectroscope. — This  instrument  consists  of 
two  small  telescopes,  with  a  prism  mounted  between 
their  object-glasses  (Fig.  106).  The  rays  of  light  enter 
through  a  narrow  slit  at  A,  and  are  rendered  parallel 
by  the  object-glass.  They  then  pass  through  the 
prisms  at  C,  are  separated  into  the  different  colors, 
and,  entering  the  second  telescope  at  D,  fall  upon  the 


Fig.  106. 


eye  at  B.  A  third  telescope  is  sometimes  attached, 
which  contains  a  minutely-accurate  scale  for  meas- 
uring the  distances  of  the  lines.  In  addition,  a 
mirror  may  throw  in  at  one  side  of  the  slit  a  ray  of 
sunlight  or  starlight,  and  so  we  can  compare  the 
spectrum  of  the  sunbeam  with  that  of  any  flame  we 
desire. 


260 


THE  SIDEREAL  SYSTEM. 


Revelations  of  the  Spectroscope  Concerning 
THE  Sun. — The  spectrum  of  the  sunbeam  is  not  con- 
tinuous, but  is  crossed  by  a  large  number  of  dark 
lines,  called,  from  their  discoverer,  Fraunhofer's 
lines.  It  is  therefore  concluded  that  the  sun's  light 
is  of  the  third  class  just  named,  and  that  it  is  pro- 
Fig.  106. 


A  Spectruscope. 


duced  by  the  vivid  light  of  a  highly  heated  body 
shining  through  a  flame  full  of  volatilized  sub- 
stances. 

But  not  only  does  spectrum  analysis  thus  shed  light 
on  the  physical  constitution  of  the  sun,  but  these 
lines  are  so  distinctive,  so  marked  and  varied,  that 
the  elements  of  which  the  sun  is  composed  may  be 
discovered.*  Thus,  for  example,  iron  gives  a  spec- 
trum of  some  450   lines,  differing  in  intensity  and 

*  The  following  twenty-two  elements  have  been  detected  :  sodium,  calcium,  liarium. 
magnesium,  iron,  chromium,  nickel,  cobalt,  hydrogen,  manganese,  aluminium,  titanium, 
palladium,  vanadium,  molybdenum,  strontium,  lead,  uranium,  cerium,  strontium, 
cadmium,  oxygen,  and  a  probability  of  several  more,  such  as  carbon,  silver,  tin,  etc. 


CELESTIAL  CHEMISTRY.  261 

relative  length.  These  are  bright  when  iron  vapor 
is  burning,  and  dark  when  white  light  is  passed 
through  such  burning  vapor.  In  the  solar  spectrum 
we  have  such  a  coincidence  of  dark  lines,  as  to  make 
the  conclusion  irresistible  that  iron  is  contained  in 
the  sun's  atmosphere.* 

Stars  are  Suns. — The  same  method  of  analysis  has 
been  applied  to  the  stars.  The  spectra  are  marked 
by  dark  lines.  Their  constitution  is  therefore  like 
our  sun,  and  they  also  exhibit  familiar  elements. 
Betelgeuse,  for  example,  contains  many  substances 
known  to  us,  but,  as  is  thought,  no  hydrogen.  What 
a  world  that  must  be  without  water  !  We  thus  trace 
in  the  faintest  star  that  trembles  in  the  measureless 
depths  of  space,  the  elements  that  compose  the  com- 
mon objects  of  our  own  life.  We  know  that  we  are 
akin  to  nature  everywhere, — a  part  of  a  system  vast 
as  the  universe. 

The  Motion  of  a  Star  may  be  resolved  into  two 
components  :  one  representing  its  motion  at  right 
angles,  and  the  other  its  motion  parallel  to  the  line 
of  vision.  The  former  component  can  be  determined 
by  the  telescope  ;  the  latter  is  revealed  by  the  spectro- 

*  Recent  researches  in  spectroscopy  present  important  problems.  On  elevating  the 
temperature,  it  has  been  found  that  not  only  the  lines  of  the  spectrum  of  a  substance 
vary,  but  new  ones  appear.  Certain  substances  have  apparently  common  lines.  A 
molecule  containing  a  few  atoms  gives  a  line-spectrum ;  increase  the  number  of  atoms 
and  it  presents  a  finted-spectrum  (i.  e.,  one  composed  of  bands,  each  made  up  of  lines, 
and  having  a  sharp  boundary  on  one  side  but  fading  away  on  the  other) ;  increase  the 
number  yet  more,  and  it  yields  a  continuous  sx)ectrum.  New  queries  have  therefore 
arisen  in  Solar  Physics.  How  many  atoms  are  there  really  in  a  specified  molecule  t 
What  is  the  meaning  of  certain  unfamiliar  lines  seen  in  the  solar  spectrum  ?  Why  do  we 
not  detect  in  the  sun  many  of  those  substances  that  form  so  large  a  part  of  the  earth's 
crust?  Lockyer  supposes  that  the  so-called  elements  are  really  compounds  whose  mol- 
ecules may  be  "dissociated"  by  intense  heat,  so  that  in  the  sun  we  see  only  the  germs 
of  our  familiar  chemical  forms.  Bead  Lockj^er's  "  Spectrum  Analysis,"  and  Young's 
t'fheSun." 


262  THE   SIDEREAL  SYSTEM, 

scope.  If  the  star  is  moving  towards  us,  the  number 
of  vibrations  producing  any  color  will  be  increased, 
and  hence  the  dark  lines  corresponding  to  that  color 
in  the  spectrum  will  be  pushed  beyond  its  usual 
place  toward  the  violet  end ;  if  going  from  us,  the 
number  will  be  decreased,  and  the  dark  lines  be 
pushed  toward  the  red  end  of  the  spectrum.*  The 
amount  of  displacement  once  determined,  the  velo- 
city of  the  star  can  be  reckoned  by  means  of  well- 
known  laws  of  optics. 

Spectra  of  Nebulae, — Instead  of  being  marked 
with  dark  lines,  as  are  the  si)ectra  of  the  stars,  many 
of  the  nebulae  exhibit  bright  lines.  Their  spectra 
are,  therefore,  of  the  2nd  kind.  This  proves  such 
nebulae  to  consist,  not,  like  the  stars,  of  an  intensely- 
heated  nucleus  shining  through  a  luminous  atmos- 
phere, but  of  a  glowing  mass  of  gas.f  Out  of  60 
nebulse  examined  by  Mr,  Huggins,  20  exhibited  the 
bright  lines  belonging  to  the  gases,  and  all  contained 
nitrogen. 

The  Solar  Flames,  which  were  formerly  seen  only 
during  an  eclipse,  can  now  be  examined  by  means  of 
the  spectroscope,  at  any  time.  J    The  sun  has  thus 

*  The  same  result  is  produced  in  the  case  of  sound.  The  whistle  of  an  approaching 
train  sounds  shriller  than  when  it  is  receding.     See  Physics,  p.  133, 

t  Tlie  Dumb-bell  nebula  is  said  to  emit  a  light  only  about  one  twenty-thousandth 
part  that  of  a  common  wax-candle.  If  this  matter  be  a  "  sun-germ,"  how  immensely 
must  it  become  condensed  before  its  rushlight  glimmering  can  rival  the  dazzling  bril- 
liancy of  even  our  own  sun  ! 

J  "  The  red  portion  of  the  spectrum  will  stretch  athwart  the  field  of  view  like  a  scarlet 
ribbon  with  a  darkish  tend  across  it ;  and  in  that  band  will  appear  the  prominences, 
like  scarlet  clouds,  so  like  our  own  terrestnal  clouds,  indeed,  in  form  and  texture,  that 
the  resemblance  is  quite  startling.  One  might  almost  think  he  was  looking  out 
through  a  partly-opened  door  upon  a  sunset  sky,  e.vcept  that  there  is  no  variety  or  con- 
trast of  color  ;  all  the  cloudlets  are  of  the  same  pure  scarlet  hue.  Along  the  edge  of  the 
op)ening  is  seen  the  chromospliere,  more  brilliant  than  the  clouds  which  rise  from  it  or 
float  above  it.  and^  for  the  most  i>art,  made  up  of  minute  tongues  and  filamente. "—youn^. 


TIME.  263 

been  found  to  be  a  sea  of  fire  swept  by  the  most  vio- 
lent storms.*  Flames  travel  over  its  surface  with  a 
velocity  of  which  we  can  form  no  conception  ;  "one 
jet  shot  out  80,000  miles  and  disappeared  in  ten  min- 
utes." Young  describes  a  protuberance  that  reached 
the  enormous  height  of  350,000  miles  and  then  faded 
entirely  away,  all  within  two  hours. 


V.    TIME. 

Sidereal  Time. — A  sidereal  day  is  the  exact  interval 
of  time  in  which  the  earth  rotates  on  its  axis.  It  is 
found  by  marking  two  successive  passages  of  a  star 
across  the  meridian  of  any  place.  This  is  so  abso- 
lutely uniform,  that,  as  recent  investigations  seem 
to  show,  the  length  of  the  sidereal  day  has  not  varied 
more  than  gV  ^f  ^  second  in  2,400  years,  (note,  p.  89). 

The  sidereal  day  is  divided  into  twenty-four  equal 
portions,  which  are  called  sidereal  hours,  and  each 
of  these  hours  into  sixty  portions,  termed  sidereal 
minutes,  etc. 

Astronomical  clocks  are  regulated  to  keep  si- 
dereal time.  The  day  commences  when  the  vernal 
equinox  is  on  the  meridian.  Therefore,  the  time  by 
a  sidereal  clock  does  not  point  out  the  hour  of  the 
ordinary  day.  It  indicates  only  how  long  it  is  since 
the  vernal  equinox  crossed  the  meridian,  and  thus 
shows  the  right  ascension  of  any  star  which  may 

*  Such  a  storm  "  coming  down  upon  us  from  the  north  would  in  30  seconds  after  it 
had  crossed  the  St.  Lawrence  be  in  the  Gulf  of  Mexico,  carrying  with  it  the  whole  sur- 
face of  the  continent  in  a  mass,  not  of  ruin  simply,  but  of  glowing  vapor,  in  which  the 
vapors  arising  from  the  dissolution  of  the  materials  composing  the  cities  of  Boston,  New 
7ork;  and  Chicago  would  be  mixed  in  a  single  undistinguishable  cloud."— Newoomb, 


364  THE  SIDEREAL  SYSTEM. 

happen  to  be  on  the  meridian  at  that  moment.  The 
hours  of  the  clock  are  easily  reduced  to  degrees  (p. 
28).  The  astronomer  always  reckons  the  hour  of  the 
day  consecutively  up  to  twenty-four. 

Solar  Time. — A  solar  day  is  the  interval  between 
two  successive  passages  of  the  sun  across  the  meri- 
dian of  any  place.  If  the  earth  were  stationary  in 
its  orbit,  the  solar  day  would  be  of  the  same  length 
as  the  sidereal ;  but,  while  the  earth  is  turning 
around  on  its  axis,  it  is  going  forward  at  the  rate  of 
360°  in  a  year,  or  about  1°  per  day.  When  the  earth 
has  made  a  complete  rotation,  it  must  therefore 
perform  a  part  of  another  rotation  through  this 
additional  degree,  in  order  to  bring  the  same  meri- 
dian vertically  under  the  sun. 

One  degree  of  diurnal  rotation  is  equal  to  about 
four  minutes  of  time.  Hence  the  solar  day  is  four 
minutes  longer  than  the  sidereal  day.  For  the  con- 
venience of  society,  it  is  customary  to  call  the  solar 
day  24  hours  long,  and  make  the  sidereal  day  only 
23  hr.  56  min.  4  sec.  in  length,  expressed  in  mean 
solar  time.  A  sidereal  day  being  shorter  than  a 
solar  one,  the  sidereal  hours,  minutes,  etc.,  are 
shorter  than  the  solar  ;  24  hours  of  mean  solar  time 
being  equal  to  24  hr.  3  min.  56  sec.  of  sidereal  time. 

From  what  has  been  said,  it  follows  that  the  earth 
makes  366  rotations  around  its  axis  in  365  solar  days. 

Mean  Solar  Time. — The  solar  days  are  of  unequal 
length.  To  obviate  this  difficulty,  astronomers  sup- 
pose a  mean  sun  moving  through  the  equator  of  the 
heavens  (which  is  a  circle  and  not  an  ellipse)  with  a 
perfectly  uniform  motion.     When  this   mean  sujj 


TIME.  265 

passes  the  meridian  of  any  place,  it  is  mean  noon ; 
and  when  the  true  sun  is  in  the  same  position,  it  is 
apparent  noon.  This  mean  day  is  the  average  length 
of  the  solar  days  in  the  year.  The  clocks  in  common 
use  are  regulated  to  keep  mean  time.*  V/hen  it  is 
twelve  by  the  clock,  the  sun  may  be  either  a  little 
past  or  a  little  behind  the  meridian. 

The  difference  between  sun-time  (apparent  solar- 
time)  and  clock-time  (mean  time)  is  called  the 
*^ Equation  of  time.''  This  is  the  greatest  about  the 
first  of  November,  when  the  sun  is  over  sixteen  and 
a  quarter  minutes  in  advance  of  the  clock.  The  sun  is 
the  slowest  about  February  10th,  when  it  is  about 
fourteen  and  a  half  minutes  behind  mean  time. 

Mean  and  apparent  time  coincide  four  times  in  the 
year — namely,  April  15th,  June  14th,  September  1st, 
and  December  24th.  On  these  days,  the  noon-mark 
on  the  sun-dial  coincides  with  twelve  o'clock. 

The  Sun-Dial. — The  apparent  time  of  the  dial  may 
be  readily  changed  to  mean  time,  by  adding  or  sub- 
tracting the  number  of  minutes  given  in  the  almanac 
for  each  day  in  the  year,  under  the  heading  "sun 
slow"  or  "sun  fast."  A  noon-mark  is  thus  a  very 
convenient  method  of  regulating  a  timepiece,  f 

♦  In  France,  until  1816,  apparent  time  was  used  ;  and  the  confusion  was  so  g^eat, 
that  Arago  relates  how  the  town  clocks  would  differ  thirty  minutes  in  striking  the  same 
hour.  As  the  time  varied  every  day,  no  watchmaker  could  regulate  a  watch  or  clock  to 
keep  it 

t  The  following  manner  of  obtaining  one  without  a  transit  instrument  may  be  use- 
ful. Select  a  level  hard  surface  which  is  exposed  to  the  sun  from  about  9  a.  m.  to  4  p.  m. 
Upon  this  carefully  describe,  with  compasses,  a  circle  of  eight  or  ten  inches  in  diameter. 
Take  a  piece  of  heavy  wire,  six  or  eight  inches  in  length,  one  end  of  which  is  sharpened. 
Drive  this  perpendicularly  into  the  center  of  the  circle,  leaving  it  just  high  enough  to 
allow  the  extreme  end  of  its  shadow  to  fall  upon  the  circle  about  9J  or  10  a.  m.  Mark 
this  point,  and  also  the  place  where  the  shadow  touches  the  circle  in  the  afternoon. 
Take  a  point  half-way  between  the  two,  and,  drawing  a  line  from  that  to  the  center  of 
the  circle,  it  will  be  the  meridian  liiie,  or  noou-mark. 


266 


THE  SIDEREAL  SYSTEM. 


Why  the  Solar  Days  are  of  Unequal  Length- — There 
are  two  reasons  for  this, — the  unequal  orbital  motion 
of  the  earth,  and  the  obliquity  of  the  ecliptic.  First : 
the  orbit  of  the  earth  is  an  ellipse  ;  and  thus  the  ap- 
parent yearly  motion  of  the  sun  along  the  ecliptic  is 
variable.  In  perihelion,  in  January,  the  sun  appears 
to  move  eastward  daily  1°  1'  9'  .9  ;  while  at  aphelion, 
in  July,  only  57'  11".  5.  As  the  earth  in  its  diurnal 
motion  rotates  uniformly  from  west  to  east,  and  the 
sun  passes  eastward  irregularly,  this  must  produce  a 
corresponding  variation  in  the  length  of  the  solar 
day.  The  sun,  therefore,  comes  to  the  meridian 
sometimes  earlier  and  sometimes  later  than  the 
mean  noon,  and  they  agree  only  at  perihelion  and 
aphelion. 

Second  :  as  we  have  just  seen,  the  mean  sun  is  sup- 
posed to  move  in  a  circle  and  not  an  ellipse.     This 

would  make  the 
motion  along 
the  ecliptic  uni- 
form, but  the 
obliquity  of  the 
ecliptic  would 
still  cause  an  ir- 
regularity in  the 
length  of  the 
day.  The  mean 
sun  is  therefore  supposed  to  pass  along  the  equi- 
noctial, which  is  perpendicular  to  the  earth's  axis  : 
while  the  ecliptic  is  inclined  to  it  23"  27'.  Let  A 
represent  the  vernal  equinox  ;  I,  the  autumnal ;  AEI, 
the  ecliptic  ;  AI,  the  equinoctial ;  PK,  PL,  PM,  etc., 


TIME.  26? 

meridians.  Let  the  distances  AB,  BC,  CD,  etc.,  be 
equal  arcs  of  the  ecliptic,  which  are  passed  over  by  the 
sun  in  equal  times.  Next,  on  the  equinoctial,  mark 
off  distances  Aa,  ah,  he,  etc.,  equal  to  AB,  BC,  etc. 
These  are  equal  arcs  of  right  ascension,  or  hour- 
circles,  through  which  the  earth,  rotating  from 
west  to  east,  passes  in  equal  times.  Now,  meridians 
drawn  through  these  divisions,  would  not  agree  with 
those  drawn  through  equal  divisions  on  the  ecliptic. 
Hence,  a  sun  moving  along  the  ecliptic,  which  is  in- 
clined, would  not  make  equal  days,  even  though  the 
ecliptic  were  a  perfect  circle. 

Let  us  see  how  the  mean  and  apparent  solar  days 
would  compare.  Let  the  real  sun  pass  in  its  east- 
ward course  from  A  to  B  in  a  certain  time  ;  the  mean 
sun  moving  the  same  distance  would  reach  the  point 
a,  since  the  latter  travels  on  the  base  and  the  former 
the  hypothenuse  of  a  triangle.  The  earth,  rotating 
from  west  to  east,  would  cause  the  real  sun  to  cross 
any  meridian  earlier  than  the  mean  sun  ;  hence,  ap- 
parent time  would  be  faster  than  clock-time.  By 
holding  the  figure  up  above  us  toward  the  heavens, 
we  can  see  how  a  westerly  sun  would  cross  the  meri- 
dian earlier  than  an  easterly  one.  Following  the 
same  reasoning,  we  can  see  that  at  the  solstice,  solar 
and  mean  time  would  agree  ;  while  beyond  that  point 
the  mean  time  would  be  faster. 

The  Civil  Day  is  the  mean  solar  day.  It  extends 
from  midnight  to  midnight.*    The  method  of  divid- 

*  Until  recently,  very  many  nations  terminated  one  day  and  commenced  the  next  at 
sunset.  Under  this  plan,  10  o'clock  on  one  day  would  not  mean  the  same  as  10  o'clock 
on  another  day.  The  Puritans  commenced  the  day  at  6  p.  m.  The  Babylonians,  Per- 
sians, and  Assyrians  began  the  day  at  sunrise. 


368  THE  SIDEREAL  SYSTEM. 

ing  the  day  into  two  portions  of  twelve  hours  each, 
is  said  to  have  been  adopted  by  Hipparchus,  150 
years  b.  c,  and  is  now  in  use  over  the  civilized 
world.  The  astronomical  method  of  reckoning  the 
hours  consecutively  up  to  twenty-four  is  much  more 
convenient,  and  is  therefore  coming  into  general 
favor.    The  names  of  the  days  are  derived  as  follows : 

1.  Dies  Solis Latin  .  • .  .Sun's  day. 

2.  Dies  Lunse . . . .     *'     ...  Moon's  day. 

3.  Tius  daeg. Saxon . . .  .Tius's  day. 

4.  Wodnes  daeg...     "     ...  .Woden's  day. 

5.  Thurnes  daeg..     "       . .  .Thor's  day. 

6.  Friges  daeg "     ...  .Friga's  day. 

7.  Dies  Satumi.... Latin  . . .  .Saturn's  day. 

The  Year. — The  sidereal  year  is  the  interval  of  a 
complete  revolution  of  the  earth  about  the  sun,  meas- 
ured by  a  fixed  star.  It  comprises  365  d.,  6  hr.,  9 
min.,  9.6  sec.  of  mean  solar  time.  The  mean  solar 
year  (tropical  year)  is  the  interval  between  two  suc- 
cessive passages  of  the  sun  through  the  vernal  equi- 
nox. It  comprises  365  d.,  5  hr.,  48  min.,  46.7  sec.  If 
the  equinoxes  were  stationary,  there  would  be  no 
difference  between  the  sidereal  and  the  tropical  year. 
As  the  equinoxes  retrograde  along  the  ecliptic  50"  of 
space  annually,  the  former  is  20  min.,  20  sec.  longer. 

The  anomalistic  year  is  the  interval  between  two 
successive  passages  of  the  earth  through  its  perihe- 
lion, which  moves  eastward  about  11".  8  annually.  It 
is  4  min.,  40  sec.  longer  than  the  sidereal  year. 

The  Ancient  Year. — The  ancients  ascertained  the 
length  of  the  year  by  means  of  the  gnomon.  This 
was  a  perpendicular  rod  standing  on  a  smooth  plane 


TIME.  269 

on  wliicli  was  a  meridian  line.  When  the  shadow 
cast  on  this  line  was  the  shortest,  it  indicated  the 
summer  solstice;  and  when  it  was  the  longest,  the 
winter  solstice.  The  number  of  days  required  for 
the  sun  to  pass  from  one  solstice  back  to  it  again  de- 
termined the  length  of  the  year.  This  they  found  to 
be  365  days.  As  that  is  nearly  six  hours  less  than 
the  true  solar  year,  dates  were  soon  thrown  into  con- 
fusion. If,  at  a  certain  date,  the  summer  solstice 
occurred  on  June  20th,  in  four  years  it  would  fall 
on  the  21st;  and  thus  it  would  gain  one  day  every 
four  years,  until  in  time  the  summer  solstice  would 
happen  in  the  winter  months. 

Julian  Calendar. — Julius  Csesar  first  attempted  to 
make  the  calendar  year  coincide  with  the  motions  of 
the  sun.  By  the  aid  of  Sosigenes,  an  Egyptian 
astronomer,  he  devised  a  plan  of  introducing  every 
fourth  year  a  leap-year,  which  should  contain  an 
extra  day.  This  was  termed  a  bissextile  year,  since 
the  sixth  (sextilis)  day  before  the  kalends  (first  day) 
of  March  was  then  counted  twice. 

Grregorian  Calendar. — Though  the  Julian  calen- 
dar was  nearly  perfect,  it  was  yet  somewhat  defec- 
tive. It  considered  the  year  to  consist  of  3G5^  days, 
which  is  11  minutes  in  excess.  This  accumulated 
year  by  year,  until  in  1582  the  difference  amounted 
to  ten  days.  In  that  year,  the  vernal  equinox  oc- 
curred on  the  11th  of  March,  instead  of  the  21st. 
Pope  Gregory  undertook  to  reform  the  anomaly,  by 
dropping  ten  days  from  the  calendar  and  ordering 
that  thereafter  only  centennial  years  which  are 
divisible  by  400  should  be  leap-years.    The  Gregorian 


270  TflE  SIDEREAL  SYSTEM. 

calendar  was  generally  adopted  in  Catholic  countries. 
Protestant  England  did  not  accept  the  change  until 
1752.  The  difference  had  then  amounted  to  11  days. 
These  were  suppressed  and  the  3rd  of  September  was 
styled  the  14th.*  Dates  reckoned  according  to  the 
Julian  calendar  are  termed  Old  Style  (O.S.);  and 
those  according  to  the  Gregorian  calendar,  New 
Style  (N.S.). 

Commencement  of  the  Year. — The  Jews  began 
their  civil  year  with  the  autumnal  equinox  ;  but 
their  ecclesiastical  year,  with  the  vernal  equinox. 
When  Caesar  revised  the  calendar,  the  Romans  com- 
menced the  year  with  the  winter  solstice  (Dec.  22)^ 
and  it  is  probable  he  did  not  intend  to  change  it 
materially.  He  ordered  it  to  date  from  January  1,  in 
order  that  the  first  year  of  his  new  calendar  should 
begin  with  the  day  of  the  new  moon  immediately 
succeding  the  winter  solstice. 

The  Earth  our  Timepiece. — The  measure  of  time 
is,  as  we  have  just  seen,  the  length  of  the  mean 
day.  This  is  estimated  from  the  length  of  the 
sidereal  day.  Hence,  the  standard  for  time  is  the 
rotation  of  the  earth  on  its  axis.  All  weights  and 
measures  are  based  on  time.  An  ounce  is  the  weight 
of  a  given  bulk  of  distilled  water.    This  is  measured 

*  This  sweeping  change  was  received  in  England  with  great  dissatisfaction.  Prof 
De  Morgan  narrates  the  following :  "  A  worthy  couple  in  a  conntrj'  town,  scandalized  by 
the  change  of  the  calendar,  continued  for  many  years  to  attempt  the  obsen'ance  o( 
Good  Friday  on  the  old  day.  To  this  end  they  walked  seriously  and  in  full  dress  to  the 
church  door,  on  which  the  gentleman  rapi)ed  with  his  stick.  On  finding  no  admittance, 
they  walked  as  seriously  back  again  and  read  the  service  at  home.  There  was  a  wide- 
spread superstition  that,  when  Christmas  day  began,  the  cattle  fell  on  their  knees  In  their 
stables.  It  was  asserted  that,  refusing  to  change,  they  continued  their  prostrations 
according  to  the  Old  Style.  In  England,  the  members  of  the  Government  were  mobbed 
in  the  streets  by  the  crowd,  which  demanded  the  eleven  days  of  which  they  had  been 
illegally  deprived." 


CELESTIAL  MEASUREMENTS.  2tl 

by  cubic  inches.  The  inch  is  a  definite  part  of  the 
length  of  a  pendulum  which  vibrates  seconds  in  the 
latitude  of  London.  Arago  remarks,  a  man  would 
be  considered  a  maniac  who  should  speak  of  the  in- 
fluence of  Jupiter's  moons  on  the  cotton  trade.  Yet 
there  is  a  connection  between  these  incongruous 
ideas.  The  navigator,  travelling  the  waste  of  waters 
where  there  are  no  paths  and  no  guide-boards,  may 
reckon  his  longitude  by  the  eclipses  of  Jupiter's 
moons,  and  so  decide  the  fate  of  his  voyage.  We 
can  easily  see  how  the  rotation  of  the  earth  on  its 
axis  influences  the  cost  of  a  cup  of  tea. 


VI.    CELESTIAL     MEASURE- 
MENTS. 

Many  persons  read  the  enormous  figures  which 
indicate  the  distances  and  dimensions  of  the  heaven- 
ly bodies  with  a  questioning,  indefinite  idea,  entirely 
unlike  the  feeling  of  certainty  with  which  they  read 
of  the  distance  between  two  cities,  or  the  number 
of  square  miles  in  a  certain  State.  Many,  too,  imag- 
ine that  celestial  measurements  are  so  mysterious  in 
themselves  that  no  common  mind  can  hope  to  grasp 
the  methods.  Let  us  attempt  the  solution  of  a  few 
of  these  problems. 

1st,  To  Find  the  Distances  of  the  Planets  from 
the  Sun.— In  Fig.  108,  E  represents  the  earth;  ES, 
the  earth's  distance  from  the  sun;  V,  the  planet 
Venus;  and  VES,  the  angle  of  elongation  (a  right- 
angled  triangle).     It  is  clear  that,  as  Venus  swings 


272  THE  SIDEREAL  SYSTEM. 

apparently  east  and  west  of  the  sun,  this  angle  may 
be  easily  measured  ;  also,  that  it  will  be  the  greatest 
when  Venus  is  in  aphelion  and  the  earth  in  peri- 
helion at  the  same  time,  for  then  VS  will  be  the 
longest  and  ES  the  shortest.  Now  in  every  right- 
Fig.  108.  angled  triangle  the  propor- 

tion between  the  hypothe- 
nuse,  ES,  and  the  side  op- 
posite, VS,  changes  as  the 
angle  at  E  varies,  but  with 
the  same  angle  remains  the 
same  whatever  may  be  the 
length  of  the  lines  them- 
selves. This  proportion  be- 
tween the  hypothenuse  and 

Comparative  Distance  of  Venus  and  the     the  sido    OppOSitC    any  angle 

is  termed  the  sine  of  that 
angle.  Tables  are  published  containing  the  sines  for 
all  angles.  In  this  way,  the  mean  distance  of  Venus 
is  found  to  be  jVn  that  of  the  earth;  Mars,  f  times; 
Jupiter,  5|  times,  etc.* 

2iid.  To  Measure  the  Moon's  Distance  from  the 
Earth. — (1.)  The  Ancient  Method.— As  the  moon's 
distance  is  so  much  less  than  that  of  the  other 
heavenly  bodies,  it  is  measured  by  the  earth's  semi- 
diameter.  The  method,  an  extremely  rough  one, 
which  was  in  use  among  the  ancients,  was  something 

•  If  the  pupil  haa  studied  Trigonometry,  lie  may  apply  here  the  simple  proportion — 
ES  :  VS  : :  Radius  :  Sine  of  47°  15"  =  greatest  elongation  of  Venus 

The  same  result  would  be  obtained  by  the  use  of  Kepler's  third  law  ;  and  on  page 
19,  we  saw  how  the  distances  of  the  planets  themselves  could  be  determined  by  the 
periodic  times,  if  the  distance  of  the  earth  from  the  sun  is  first  known.  So  that  when 
we  have  accurately  determined  the  sun's  distance  from  us,  we  can  then  decide  by  either 
of  the  methods  named  the  distance  of  all  the  planets.  Indeed  the  sun's  distance  is,  as 
already  remarked,  the  "  foot-rule  "  for  measuring  all  celestial  distances. 


CELESTIAL  MEASUREMENTS.  273 

like  the  following :  In  an  eclipse  of  the  moon,  that 
body  passes  through  the  earth's  shadow  in  about 
four  hours.  If,  then,  in  four  hours,  the  moon  travels 
along  its  orbit  a  distance  equal  to  the  diameter 
of  the  earth,  in  twenty-four  hours  it  would  pass  over 
six  times,  and  in  a  lunar  month  (about  thirty  days) 
one  hundred  and  eighty  times,  that  distance.  The 
circumference  of  the  lunar  orbit,  then,  must  be  one 
hundred  and  eighty  times  the  diameter  of  the  earth. 
The  ancients  supposed  the  heavenly  orbits  to  be 
circles,  and,  as  the  diameter  of  a  circle  is  about  ^  of 
the  circumference,  they  deduced  the  diameter  of  the 
moon's  orbit  as  120  times,  and  the  distance  of  the 
moon  from  the  earth  as  60  times,  the  semi-diameter 
of  the  earth. 

(2.)  Modern  Method  by  the  Lunar  Parallax. — 
Under  the  head  of  parallax,  we  saw  how,  in  common 
life,  we  obtain  a  correct  idea  of  the  distance  of  an 
object  by  means  of  our  two  eyes.  We  proved  that 
one  eye  alone  gives  no  notion  of  distance.  Just, 
then,  as  we  use  two  eyes  to  find  how  far  from  us  an 
object  is,  so  the  astronomer  uses  two  astronomical 
eyes,  or  observatories,  located  as  far  apart  as  pos- 
sible, to  find  the  parallax  of  a  heavenly  body.  In 
Fig.  109,  M  represents  the  moon ;  G,  an  observatory 
at  Greenwich  ;  and  C,  another  at  the  Cape  of  Good 
Hope.  At  the  former,  the  distance  from  the  north 
pole  to  the  center  of  the  moon,  measured  on  a 
meridian  of  the  celestial  sphere,  is  .found  to  be  108°. 
At  the  latter  station,  the  distance  from  the  south 
pole  to  the  moon's  center  is  measured  in  the  same 
Wdij,  aijd  foun(J  to  be  73^°.    The  sum  of  these  angles 


274  THE  SIDEREAL   SYSTEM. 

is  1811^''.  Now,  the  entire  distance  from  the  north 
pole  around  to  the  south  pole,  measured  on  a  meridian, 
can  be  only  half  a  great  circle,  or  180°.  This  differ- 
ence of  1|°  must  be  the  difference  in  the  position  of 
the  moon,  as  seen  from  the  two  observatories.  For 
the  observer  at  the  former  station  will  see  the  moon 
projected  on  the  celestial  sphere  at  G',  and  in  meas- 
uring its  distance  from  the  north  pole  will  measure 
an  arc  bQ'  further  than  if  he  were  located  at  E,  the 
center  of  the  earth.  The  observer  at  the  latter  sta- 
tion will  see  the  moon  projected  on  the  celestial  sphere 
at  C,  and  in  measuring  its  distance  from  the  south 
pole  will  measure  an  arc  bC  more  than  if  he  were 
located  at  E,  the  center  of  the  earth.  The  sum  of 
bG'  and  bC  =  G'C  is  the  difference  in  the  position  of 
the  moon  as  seen  from  the  two  stations.  In  other 
words,  it  is  the  moon's  parallax.  The  arc  GC, 
measures  the  angle  C'MG';  that  angle  is  equal  to  the 
opposite  angle  GMC  =  1^°.  Now,  in  the  four-sided 
figure  GECM,  the  sides  GE  and  CE  are  both  equal 
radii  of  the  earth  =  3,956  miles;  while  the  distance 
from  G  to  C  is  the  difference  in  the  latitude  of  the 
two  places.  The  angles  ZGM  and  Z'CM,  being  the 
zenith  distances  of  the  moon,  are  known,  and  so  the 
angles  MGE  and  MCE  are  easily  found.  EM,  the 
moon's  distance  from  the  center  of  the  earth,  is 
thus  readily  computed  by  a  simple  trigonometrical 
formula. 

(3.)  The  Horizontal  Parallax  of  the  Moon  is 
most  commonly  found  by  estimating  its  distance, 
not  from  the  north  and  south  poles,  as  just  explained 
under  the  general  meaning  of  the  term  parallax,  but 


CELESTIAL   MEASUREMENTS. 


275 


from  a  fixed  star.  The  moon's  horizontal  parallax  is 
now  estimated  at  57',  which  makes  its  distance  about 
sixty  times  the  earth's  semi-diameter  (p.  273).* 


Fig.  109. 


P'  z 

Measuring  Moon's  Distance  from  the  Earth. 

3.  To  Find  the  Sun's  Distance  from  the  Earth. — 
This  might  be  estimated  by  obtaining  the  solar  paral- 
lax in  the  same  manner  as  the  lunar  parallax.  It 
would  be  necessary  only  to  take  the  sun's  distance 
from  the  north  and  south  poles  respectively  at  Green- 
wich and  the  Cape  of  Good  Hope,  and  then  subtract- 
ing 180°  from  the  sum  of  the  two  angular  distances, 
the  remainder  would  be  the  solar  parallax.  The  dif- 
ficulty in  this  method  lies  in  the  fact  that  when  the 
sun  shines  the  air  is  full  of  tremulous  motion.  This 
increases  refraction — that  plague  of  all  astronomical 
calculations — to  such  an  extent  that  it  becomes  im- 

*  In  figure  110,  let  S  represent  the  moon,  sun,  or  any  other  heavenly  body  ;  AB,  the 
semi-diameter  of  the  earth  ;  and  ASB,  the  "horizontal  parallax"  of  the  body.  Then,  by 
tthe  following  trigonometrical  formula,  the  distance  fro.ip  thp  earth  may  be  easily  p*lci)r 
Jated— AS  :  AB  : :  Radius  :  Sin  of  AS? 


276  THE   SIDEREAL  SYSTEM. 

possible  to  calculate  so  small  an  angle  with  any 
accuracy.  Neither  can  the  parallax  be  estimated,  as 
in  the  case  of  the  moon,  bj'^  measuring  the  distance 

Fig.  110. 
B 


from  a  fixed  star,  since  when  the  sun  shines  the  stars 
near  by  are  invisible  even  in  a  telescope.  Astrono- 
mers have  therefore  been  compelled  to  resort  to  other 
methods. 

(1.)  Calculation  of  Solar  Parallax  by  Observ- 
ing Mars. — We  have  already  seen  that  the  distance 
of  Mars  from  the  sun  is  |  that  of  the  earth  from  the 
sun.  If,  therefore,  we  can  find  Mars's  distance  from 
the  earth,  we  can  multiply  it  by  three,  and  so  obtain 
the  distance  of  the  sun  from  the  earth.  In  1862,  when 
Mars  was  in  opposition,  it  came  very  near  us,  for  it 
was  in  perihelion  while  the  earth  was  in  aphelion, 
so  that  its  distance  (as  since  ascertained)  was  only 
about  34,000,000  miles.  Astronomers  at  Greenwich 
and  the  Cape,  and  at  various  American  and  European 
observatories,  calculated  the  distance  of  the  planet 
from  the  north  and  south  poles,  as  well  as  from  seve- 
ral fixed  stars,  in  the  manner  just  explained  for 
obtaining  the  lunar  parallax.  The  result  of  these 
observations  fixed  the  solar  parallax  at  8".  94,*  mak- 
ing the  sun's  distance  91,430,000  miles. 

*  3j'  the  formula  on  page  27"),  wc  have,  AS  :  AB  : :  Radius  :  Sin  8".94, 


CELESTIAL  MEASUREMENTS. 


277 


(2.)  Calculation  of  Solar  Parallax  by  Obser- 
vation OF  the  Transit  of  Venus.— In  the  figure,  let 
A  and  B  represent  the  position  of  two  observers  sta- 


Fig.  111. 


Transit  of  Vemte. 

tioned  at  opposite  sides  of  the  earth.  At  the  time  of 
the  transit,  the  one  at  A  will  see  the  planet  Venus  as 
a  round  black  spot  at  V"  on  the  sun's  disk,  while  the 
one  at  B  will  see  it  at  V.  The  distance  V'V"  is  the 
difference  in  the  position  of  Venus  as  seen  from  the 
two  stations  on  the  earth.  The  distance  AB  is  the 
diameter  of  the  earth.  The  distance  V'V"  is  as  much 
greater  than  AB  as  VV"  is  greater  than  VA.  The 
distance  of  Venus  from  the  sun  is  known,  by  Prob. 
I.,  to  be  .72  that  of  the  earth.  The  distance  of  Venus 
from  the  earth  must,  then,  be  1.00— .72 =.28.  Hence, 
VV",  the  distance  from  the  sun  to  Venus, =.72-^.28= 
2. 5  times  the  length  of  AV,  the  distance  of  Venus  from 
the  earth.  Therefore,  V'V"  is  equal  to  2^  times  AB, 
the  earth's  diameter,  or  5  times  the  solar  paral- 
lax. Knowing  the  hourly  motion  of  Venus,  it  is 
necessary  only  for  each  observer  to  find  when  the 
planet's  disk  enters  upon  and  leaves  the  sun's  disk, 
to  determine  the  length  of  the  path  (chord)  it 
traces.    A  comparison  of  the  length  and  direction 


278  THE  SIDEREAL  SYSTEM. 

of  these  chords  will  give  the  length  V  V"  in  seconds 
of  space. 

The  advantage  of  this  method  is  that,  as  the  dis- 
tance V  V"  is  two  and  half  times  that  of  AB,  an 
error  in  measuring  that  chord  affects  the  solar  par- 
allax less  than  one-fifth. 

Time  of  a  Transit  of  Venus.* — This  is  an  event  of 
rare  occurrence.  It  happens  only  at  intervals  of  8, 
105^ ;  8,  131^,  years,  &c.  Were  the  planet's  orbit  in 
the  same  plane  as  the  ecliptic,  a  transit  would  take 
place  during  each  synodic  revolution ;  but  as  it  is 
inclined  about  3|°,  the  transit  can  occur  only  when 
the  earth  is  at  or  near  one  of  the  nodes  at  the  same 
time  with  the  planet  when  in  inferior  conjunction. 
As  the  nodes  of  Venus  now  fall  in  that  part  of  the 
earth's  orbit  which  we  pass  in  the  beginning  of 
June  and  December,  transits  always  occur  in  those 
months. 

The  Transit  of  June  3rd,  1769,  excited  great  inter- 
est. King  George  III.  fitted  out  an  expedition  to 
Tahiti,  under  the  coromand  of  the  celebrated  naviga- 
tor, Capt.  James  Cook.  In  order  to  make  the  angle 
as  great  as  possible,  and  so  increase  the  length  of  the 
chords,  or  paths  of  the  planet  across  the  sun,  astron- 
omers were  sent  to  all  the  most  favorable  points  of 
observation — St.  Petersburg,  Pekin,  Lapland,  Cali 

*  Tlie  first  transit  ever  seen  was  witnessed  by  Horrox,  a  young  amateur  astronomei 
residing  near  Liverpool.  His  calculations  fixed  upon  Sunday,  Nov.  24,  1639  (0.  S.)- 
He,  however,  commenced  his  watch  of  the  sun  on  Saturday  preceding.  The  following 
day  he  resumed  his  ob3er\'ation  at  sunrise.  Tlie  hour  for  church  arriving,  he  repaired 
to  service  as  usual.  Returning  to  his  labor  Immediately  afterward,  he  says  :  "At  this 
time  an  opening  in  the  clouds,  which  rendered  the  sun  distinctly  vnsible,  seemed  as  if 
Divine  Pro\idence  encouraged  my  aspirations ;  when — oh  most  gratifying  spectacle !  the 
object  of  so  many  earnest  wishes — I  perceived  a  new  spot  of  perfectly  round  form  that 
had  just  entered  upon  the  left  limb  of  the  sun." 


CELESTIAL  MEASUREMENTS.  279 

fornia,  etc.  They  fixed  the  solar  parallax  at  8".  58, 
making  the  sun's  distance  95,393,055  miles.* 

The  transits  of  Dec.  8,  1874,  and  Dec.  6,  1882,  were 
carefully  observed  by  several  government  expedi- 
tions ;  the  results  have  not  yet  been  fully  announced. 

The  next  transits  will  happen, 

June  8 2004. 

June  6 2012. 

December  11 2117. 

December  S 2125. 

June  11 2247. 

The  transits  of  Mercury  are  more  frequent ;  but 
owing  to  the  nearness  of  the  planet  to  the  sun,  they 
are  of  little  value  in  determining  the  solar  parallax. 

Changes  in  the  Estimate  op  the  Solab  Paral- 
lax.— About  1824,  Encke  deduced  8".  58  as  the  prob- 
able result  of  the  observations  upon  the  transit  of 
1769.  This  conclusion  held  the  ground  for  nearly 
thirty  years,  and  the  corresponding  solar  distance  of 
95,293,000  miles  is  found  in  all  the  older  text-books. 
About  1860,  Le  Verrier  announced  that  he  could 
reconcile  the  theories  regarding  certain  of  the  plan- 
ets only  by  assuming  a  greater  solar  parallax.  As 
the  result  of  various  calculations,  together  with  the 
material  furnished  by  the    observations  upon  the 

♦  Le  Gentil,  sent  out  by  the  French  Academy  to  observe  the  transit  of  1761  in  the 
East  Indies,  was  prevented  from  making  liis  first  i)ort  by  the  war  with  England.  High 
winds  afterward  kept  him  out  at  sea  till  the  transit  was  over.  He  then  resolved  to  re- 
main abroad  until  after  the  transit  of  1769.  Eight  long  years  passed,  and  the  morning 
of  June  3,  1709,  dawned  bright  and  beautiful.  Le  Gentil,  with  his  instruments  all  in 
place,  was  counting  the  moments  for  the  long-awaited  transit  to  begin  ;  when,  suddenly, 
the  sky  grew  black  with  clouds,  and  a  tropical  storm,  the  first  in  days,  swept  by. 
Meantime,  Venus  came  and  went,  and  the  ill-fated  Le  Gentil  had  again  lost  the  oppor- 
tunity of  years.  Prostrated  by  his  bitter  disappointment,  it  was  two  weeks  before  he 
could  hold  his  pen  to  write  the  story  of  his  second  failure. 


280  THE  SIDEREAL  SYSTEM. 

planet  Mars  in  1862,  a  new  parallax  of  8".  94  was 
obtained.  This  has  been  accepted  by  all  until 
recently,  and  was  used  in  former  editions  of  this 
work.  It  is  now  known  to  be  too  large,  and  astron- 
omers are  making  every  effort  to  determine  this  most 
important  factor  in  celestial  measurements.  As  al- 
ready stated  on  page  36,  the  parallax  at  present  re- 
ceived is  about  8".  80,  which  represents  a  mean  solar 
distance  of  92,885,000;  in  round  numbers,  93,000,000, 
as  given  in  the  present  edition. 

The  difficulty  of  determining  the  solar  parallax 
accurately  will  be  seen,  when  one  is  told  that  the 
correction  from  the  old  value  of  8".  58  to  the  recent 
one  of  8  '.94,  was  a  change  in  the  angle  equal  to  that 
which  the  breadth  of  a  human  hair  would  make 
when  seen  at  a  distance  of  125  feet.  Yet  this  reduced 
the  estimated  distance  of  the  sun  from  95,293,000 
miles,  to  91,430,000  miles. 

4.  To  Find  the  Longitude  of  a  Place.* — (1.)  The 
Solar  Method. — If  the  sailor  can  see  the  sun,  he 
watches  it  closely  with  his  sextant ;  and  when  the 
sun  ceases  to  rise  any  higher  in  the  heavens  it  is  ap- 
parent noon.  By  adding  or  subtracting  the  equation 
of  time  (as  given  in  his  almanac),  he  obtains  the  true 
or  mean  noon.  He  then  compares  the  local  time  thus 
determined,  with  the  Greenwich  time  as  kept  by  the 

*  It  is  pleasant  to  notice  tliat  tlie  astronomer  can  preiUct  witli  the  utmost  precision. 
He  announces  that  on  such  a  year,  niontli,  day,  hour,  and  second,  a  celestial  body  will 
occupy  a  certain  position  in  the  heavens.  At  the  time  indicated,  we  point  our  telescope 
to  the  ])lace,  and,  at  tlie  instant,  true  beyond  the  accuracy  of  any  timepiece,  the  orb 
sweeps  into  view  !  A  prediction  of  the  Nautical  Almanac  is  received  with  as  much 
confidence  as  if  it  were  a  fact  contained  in  a  book  of  history.  "  On  the  trackless  ocean, 
this  book  is  the  mariner's  trusted  friend  and  counsellor;  daily  and  nightly  its  revela- 
tions bring  safety  to  ships  in  all  parts  of  the  world.  It  is  something  more  than  a  mere 
book.    It  is  ao  ever-present  manifestation  of  the  order  and  harmony  of  the  universe." 


CELESTIAL   MEASUREMENTS.  281 

ship's  chronometer.     The  difference  in  time  reduced 
to  degrees,  gives  the  longitude. 

(2.)  The  Lunar  Method.— On  account  of  the  diffi- 
culty in  obtaining  a  watch  which  will  keep  the  exact 
Greenwich  time  through  a  long  voyage,  the  moon  is 
more  generally  relied  upon  than  the  chronometer. 
The  Nautical  Almanac  is  always  published,  for  the 
benefit  of  sailors,  three  years  in  advance.  It  gives 
the  distance  of  the  moon  from  the  principal  fixed 
stars  which  lie  along  its  path,  at  every  hour  in  the 
night.  The  sailor  has  only  to  determine  with  his 
sextant  the  moon's  distance  from  any  fixed  star,  and  ■ 
then,  by  referring  to  his  almanac,  find  the  correspond- 
ing Greenwich  time.  By  comparing  this  with  the 
local  time,  and  reducing  the  difference  to  degrees, 
etc.,  he  obtains  the  longitude. 

5.  To  Find  the  Latitude  of  a  Place.— (1.)  By 
means  of  the  sextant  find  the  elevation  of  the  pole 
above  the  horizon,  and  this  gives  the  latitude  direct- 
ly.    (Fig.  35.) 

(2.)  In  the  same  manner,  determine  the  height  of 
the  sun  above  the  horizon  at  noon.  The  sun's  decli- 
nation for  that  day  (as  laid  down  in  the  almanac), 
added  to  or  subtracted  from  this,  gives  the  height  of 
the  equinoctial  above  the  horizon.  Subtract  this 
result  from  90°,  and  the  remainder  is  the  latitude. 

"  Place  an  Astronomer  on  board  a  sliip  ;  blindfold  him  ;  carry  him  by 
any  route  to  any  ocean  on  the  globe,  whether  under  the  tropics  or  in  one 
of  the  frigid  zones  ;  land  him  on  the  wildest  rock  that  can  be  found  ; 
remove  his  bandage,  and  give  him  a  chronometer  regulated  to  Greenwich 
or  Washington  time,  a  transit  instrument  Anth  the  proper  appliances,  and 
the  necessary  books  and  tables,  and  in  a  single  clear  night  he  can  tell  his 
position  mthin  a  hundred  yards  by  observations  of  the  stars. " 


282  THE  SIDEREAL  SYSTEM. 

6.  To  Find  the   Circumference   of  the  Earth. — If 

the  earth  were  a  perfect  sphere,  it  is  obvious  that 
degrees  of  latitude  would  be  of  the  same  length 
wherever  measured  on  its  surface.  Each  would  be 
^^0^  of  the  entire  circumference.  If,  however,  a  per- 
son sets  out  from  the  equator,  and  travels  along  a 
meridian  toward  either  pole,  and,  when  the  polar  star 
has  risen  in  the  heavens  one  degree  above  the  horizon, 
he  marks  the  spot,  and  then  continues  his  journey, 
marking  each  degree  in  succession,  he  will  find  that 
the  degrees  are  not  of  equal  length,  but  increase 
gradually  from  the  equator  to  the  pole.  If,  now,  the 
length  of  a  degree  be  measured  at  different  places, 
the  rate  of  variation  can  be  found,  and  then  the 
average  length  be  estimated.  Measurements  for  this 
purpose  have  been  made  in  Peru  (almost  exactly  at 
the  earth's  equator),  Lapland,  England,  France, 
India,  Russia,  etc.  So  great  accuracy  has  been  at- 
tained, that  Airy  and  Bessel,  who  have  solved  the 
problem  independently,  differ  in  their  estimate  of 
the  equatorial  diameter  but  77  yards,  or  only  y^o  of 
a  mile. 

7.  To  Find  the  Relative  Size  of  the  Planets. — The 
volumes  of  two  globes  are  proportional  to  the  cubes 
of  their  like  dimensions.  The  diameter  of  Mercury 
is  3,000  miles,  and  that  of  the  earth  7,925  ;  then, 

The  volume  of  Mercury  :  the  volume  of  the  earth  :  :  3000^  :  7925". 

The  same  principle  applied  to  the  volume  or  bulk  of 
the  sun  gives — 

The  bulk  of  the  sun  :  bulk  of  the  earth  ::  866,0003  :  7925», 


PRACTICAL   QUESTIONS.  283 

8.  To  Find  the  Diameter  of  the  Sun. — (1.)  A  very 
simple  method  is  to  hold  up  a  circular  piece  of  paper 
before  the  eye  at  such  a  distance  as  exactly  to  hide 
the  entire  disk  of  the  sun.  Then  we  have  the  pro- 
portion, 

As  dist.  of  paper  disk  :  dist.  of  sun's  disk  :  :  diam.  of  paper  d.  :  diam.  sun's  d. 

(2.)  The  apparent  diameter  of  the  sun,  as  seen 
from  the  earth,  is  about  32':  the  apparent  diameter 
of  the  earth,  as  seen  from  the  sun,  is  twice  the  solar 
parallax,  or  17". 60  (p.  36).    Thence,  the 

Ap.  diam.  of  earth  :  ap.  diam.  of  sun  :  :  real  diam.  of  earth  :  real  diam.  of  sun. 

(3.)  Knowing  the  apparent  diameter  of  the  sun, 
and  its  distance  from  the  earth,  the  real  diameter  is 
found  by  Trigonometry.  In  figure  110,  let  S  represent 
the  earth  ;  AB,  the  radius  of  the  sun ;  and  ASB,  half 
the  apparent  diameter  of  the  sun.  We  shall  then 
have  the  proportion, 

AS  :  AB  :  :  radius  :  sin.  16'  (half  mean  diam.  of  sun). 

By  a  similar  method  the  diameters  of  the  planets  are 
obtained. 


PRACTICAL    QUESTIONS. 

1.  In  what  constellation  is  Job's  Coffin  ?     The  Letter  Y  ?    The  Scalene 
Triangle  ?     The  Dipper  ?     The  Kids  ?     The  Triangles  ? 

2.  Name  some  facts  in  the  solar  system  for  which  the  nebular  hypo- 
thesis fails  to  account. 

3.  Which  is  probably  hotter,  a  yellow  or  a  red  star  ? 

4.  Are  any  of  the  stars  likely  to  collide  with  each  other  ? 

5.  Is  the  real  day  longer  or  shorter  than  the  apparent  one  ? 

6.  Do  we  ever  see  the  stars '( 


284  PRACTICAL    QUESTIONS. 

7.  What  fixed  star  is  nearest  the  earth  ? 

8.  How  often  is  Polaris  on  the  meridian  of  a  place  ? 

9.  How  do  we  know  that  the  stars  are  suns  ? 

10.  Can  a  watch  keep  apparent  time  ? 

11.  How  could  a  child  be  8  years  old  before  a  return  of  its  birthday  ? 
12    ^^"^leu  will  a  watch  and  a  sun-dial  agree  ? 

13.  "What  star  will  be  the  Pole  Star  next  after  Polaris  ? 

14.  Why  is  the  birthday  of  Washington  celebrated  on  Feb.  22,  when  he 
was  born  Feb.  11,  1732  (0.  S.)  ? 

15.  Does  the  tide  have  any  effect  on  the  length  of  the  day  ? 

16.  Will  the  Big  Dipper  always  look  as  it  does  now  ? 

17.  How  many  times  does  the  earth  turn  on  its  axis  every  year  ? 

18.  Does  the  spectroscope  tell  us  anything  concerning  the  constitution 
of  the  moon,  or  any  of  the  planets  ? 

19.  When  the  United  States  bought  Alaska  from  Russia,  the  calendar 
used  there  was  found  to  be  one  day  ahead  of  our  reckoning.  Why  was 
this  ? 

20.  "Why  do  the  dates  of  the  solstices  and  equinoxes  vary  a  day  in  differ- 
ent years  ? 

21.  Why  are  not  forenoon  and  afternoon  of  the  same  day,  as  given  in 
the  almanac,  of  equal  length  ? 

22.  In  what  part  of  the  heavens  do  the  stars  apparently  move  from  west 
to  east  ? 

23.  What  year  was  only  nine  months  and  six  days  long  ? 

24.  What  day  will  be  the  last  day  of  the  Xineteeuth  Century  ? 

25.  If  one  should  watch  tlie  sky,  on  a  winter's  evening,  from  6  v.  si.  to 
6  A.  M.,  what  portion  of  the  celestial  sphere  would  he  be  able  to  see  ? 

26.  How  do  we  know  tliat  the  moon  has  little,  if  any,  atmosphere  ? 

27.  In  Greenland,  at  what  part  of  the  year  will  the  midnight  sun  be  seen 
due  north  ? 

28.  Can  you  give  any  other  proof  of  the  rotundity  of  the  earth,  besides 
that  named  in  the  text  ? 

29.  Point  out  the  error  in  the  following  passage  from  Byron's  ' '  Darkness  " 
where  the  poet,  in  describing  the  effect  of  the  sun's  destruction,  says — 

■'  I  had  a  dream,      »      »      * 

*      *      *      which  was  not  all  a  dream, 
The  bright  sun  was  extinguislied,  and  the  stars 
Did  wander  darkling  in  the  external  space 
Ray  less  aud  pathless." 


PRACTICAL  QUESTIONS.  285 

30.  Explain  the  remark  of  the  First  Carrier  in  Scene  i,  Act  ii,  King 
Henry  IV  :  "  An't  be  not  four  by  the  day,  I'll  be  hanged  :  Charles'  wain 
is  over  the  new  chimney." 

31.  Why  does  not  the  earth  move  with  equal  velocity  in  all  parts  of  its 
orbit  ? 

32.  How  many  Jovian-years  old  are  you  ? 

33.  Why  is  the  sky  blue  ? 

34.  At  what  season  of  the  year  does  Cliristmas  occur  in  Australia  ? 

35.  What  causes  tlie  apparent  movement  of  the  sun  north  and  south  ? 

36.  On  what  part  of  the  earth  is  the  twilight  the  longest  f    The  shortest  I 

37.  Name  the  causes  which  make  our  summer  longer  than  winter. 

38.  Why  is  not  total  darkness  produced  when  a  dense  cloud  passes  be- 
tween us  and  the  sun  ? 

39.  Why  does  the  time  of  the  tide  vary  each  day  ? 

40.  Why  is  an  annular,  longer  than  a  total,  eclipse  ? 

41.  Why  is  it  colder  in  winter  than  in  summer  ' 

42.  Do  the  solar  spots  affect  our  weather  ? 

43.  Can  the  moon  be  eclipsed  in  the  day-time  ? 

44.  Why  are  the  sidereal  days  of  uniform  length  ? 

45.  Why  are  not  the  solar  days  of  uniform  length  ? 

46.  What  does  the  moon's  phases  prove  ? 

47.  Why  do  the  sun  and  moon  a})pear  flattened  when  near  the  hori- 
zon? 

48.  How  many  stars  can  we  see  with  the  naked  eye  ? 

49.  Is  there  ever  an  annular  eclipse  of  the  moon  < 

50.  "While  the  sun  rises  and  sets  365  times,  a  star  rises  and  sets  366 
times."     Explain. 

51.  How  many  moons  are  there  in  the  solar  system  ? 

52.  What  causes  the  twinkling  of  the  stars  ( 

53.  Name  some  of  the  uses  of  the  stars.  * 

*  "  To  the  astronomer,  the  fixed  stars  are  immovable  lioundary-stones  by  which  he 
determines  the  courses  of  the  wandering  lieavenly  bodies.  To  the  geographer,  they  are 
the  signal-stations  according  to  which  he  surveys  the  chart  of  the  earth  by  the  heavens. 
To  the  mariner,  they  are  the  lights  that  direct  him  over  the  dark  paths  of  the  seas.  To 
the  hunter,  the  herdsman,  the  wanderer,  they  are  a  clock.  To  the  farmer,  they  are  a 
calendar.  The  historian  finds  in  them  many  a  memorable  event  in  the  oldest  Grecian 
history.  The  poet  reads  in  them  the  charming  Grecian  mythology,  which  has  furnished 
such  rich  materials  to  dramatic  art  ;  and  every  person  of  sensibility  receives  from  them 
au  impulse  to  worship,  meditation,  and  liope." 


286  PRACTICAL  QUESTIONS. 

54.  Describe  the  methods  by  which  we  determine  the  distance  of  the  sun 
from  the  earth. 

55.  Why  do  not  the  signs  and  the  constellations  of  the  Zodiac  agree  ? 

56.  When  we  look  at  the  Xorth  Star,  how  long  since  the  light  that  enters 
our  eye  has  left  that  IxkIv  ? 

57.  In  what  diiection  does  a  comet's  tail  generally  point  ? 

58.  ^Vhat  is  the  cause  of  shooting  stars  ? 

59.  Why  docs  the  crescent  moon  appear  larger  than  the  dark  body  of  the 
moon  ? 

60.  What  is  the  i-eal  path  of  the  moon  ? 

61.  What  would  be  the  result  if  the  axis  of  the  earth  were  parallel  to  the 
plane  of  its  orbit  ? 

62.  Do  we  see  the  same  stai-s  at  different  seasons  of  the  year  ? 

63.  Why  do  we  not  jierceive  the  earth's  motion  in  space  ? 

64.  Did  the  earth  ever  shine  as  a  star  ?     Does  it  now  shine  as  a  planet  ? 

65.  What  is  the  nebular  hyjx)thesis  ? 

66.  What  is  the  cause  of  the  solar  spots  ? 

67.  Would  it  make  the  new  moon  "drier"  or  "wetter"  if  the  moon's 
path  ran  north  of,  instead  of  on,  the  ecliptic  at  the  time  of  new  moon  '. 

68.  Under  what  conditions  are  we  accustomed  to  transfer  motion  ? 

69.  ^^'^ly  do  not  the  planets  twinkle  1 

70.  Why  is  the  horizon  a  circle  ? 

71.  What  causes  are  gradually  increasing  the  length  of  the  day  ? 

72.  What  distance  does  the  moon  gain  in  her  orbit  each  year  ? 

73.  State  the  general   argument  which  renders  it  probable  that  other 
worlds  are  inhabited. 

74.  Illustrate    the    uniformity    of    Xature.     \Miat    thought    does   this 
suggest  ? 

75.  At  what  rate  are  we  traveling  through  space  ?     How  is  this  determ- 
ined? 

76.  ^^^ly  does  the  length  of  a  degree  of  latitude  increase  in  going  from 
the  equator  toward  either  pole  of  the  earth  ? 

77.  How  can  you  detect  the  yearly  motion  of  the  sun  among  the  stars  ? 

78.  Have  you  actually  traced  the  movement  of  any  one  of  the  planets,  so 
as  to  understand  its  jjecuUar  and  irregular  wandering  among  the  stars  ? 

79.  How  do  you  explain  the  varied  aspect  of  the  heavens  in  the  different 
seasons  of  the  year  ? 


PRACTICAL   QUESTIONS.  287 

80.  How  Joes  the  sjjiiiniug  of  a  top  illustrate  the  subject  of  precession  ? 

81.  Why  do  solar  eclipses  come  on  from  the  west  and  cross  to  the  east, 
while  lunar  eclipses  come  on  from  the  east  and  cross  to  the  west  !■ 

82.  Newcomb,  in  his  Astronomy,  says  that,  "  If,  when  the  moon  is  near 
the  meridian,  an  observer  could  in  a  moment  jump  from  New  York  to 
Liverpool,  keeping  his  eye  fixed  upon  that  body  he  could  see  her  apparently 
jump  in  the  opposite  direction  about  the  same  distance."     Explain. 

83.  When,  and  by  whom,  was  the  basis  of  the  calendar  we  now  use  fully 
established  ? 

84.  How  much  is  the  Russian  reckoning  of  time  ]>ehind  ours  ? 

85.  Is  there  any  gain  in  having  the  astronomical  and  the  calendar  year 
agree  ? 

86.  What  religious  festival  is  fixed  each  year  by  the  motion  of  the  moon  ? 

87.  Why  can  we,  at  different  times,  see  both  poles  of  the  planet  Mai-s  < 

88.  What  famous  astronomical  discovery  was  made  on  the  tii-st  day  of 
this  century  ? 

89.  Do  the  stars  rise  and  set  at  the  poles  ? 

90.  Name  and  locate  the  stars  of  the  first  magnitude  which  are  seen  in 
our  sky. 

91.  Name  three  bright  stars  which  lie  near  the  first  meridian. 

92.  What  events  were  transpiring  in  our  history  a  Saturnian  century  ago  ? 

93.  What  is  the  sun's  declination  at  the  winter  solstice  ?  At  the  autunmal 
equinox  ? 

94.  Will  the  u-idth  of  the  terrestrial  zones  always  remain  exactly 
as  now  ? 

95.  Is  it  always  noon  at  12  o'clock  ? 

96.  ^^^len  the  sun's  declination  is  23^  N.,  in  what  sign  is  he  then 
located,  and  what  is  his  R.  A.  ? 

97.  What  is  the  apparent  diameter  of  the  sun  ? 

98.  How  can  a  sailor  find  his  latitude  and  longitude  at  sea  ? 

99.  How  many  miles  on  the  solar  disk  represent  a  second  of  apparent 
diameter  ? 

100.   At  what  latitude  will  there  be  twilight  during  the  entire  midsummer 
Vight  ? 


Fig.  lis. 


Cambridge  Equatorial  Telescope. 


IV. 


APPENDIX 


APPENDIX. 


TABLE  ILLUSTEATING  KEPLER'S  THIRD  LAW.    (CHA>rBERS.) 

In  the  first  column  are  the  relative  distances  of 
the  planets  from  the  sun  ;  in  the  second,  the  periodic 
times  of  the  planets ;  and  in  the  third,  the  squares 
of  the  periodic  times  divided  by  the  cubes  of  the 
mean  distances.  The  decimal  points  are  omitted  in 
the  third  column  for  convenience  of  comparison. 
The  want  of  exact  uniformity  is  doubtless  due  to 
errors  in  the  observations. 


Vulcan  ? 
Mercury 
Venus  • 
Earth   - 
Mars     - 
Jupiter 
Saturn 
Uranus 
Neptune 


.143 

19.7 

1.32  716 

.38710 

87.969 

133  421 

.72333 

224.T01 

133  413 

1. 

365.256 

133  403 

1.52369 

686.979 

133  410 

5.20277 

4,a32.585 

1.33  294 

9.53858 

10,759.220 

133  375 

19.18239 

30,686.821 

133  422 

30.03627 

60,126.722 

133  413 

Arago,  speakinsr  of  Kepler's  Laws,  says :  "  These  interesting  laws,  tested  for  every 
planet,  have  been  found  so  perfectly  exact,  that  we  do  not  hesitate  to  infer  the  dis- 
tances of  the  planets  from  the  sun  from  the  duration  of  their  sidereal  periods ;  and  it 
is  obvious  that  this  method  possesses  considerable  advantages  in  point  of  exactness." 


MEASUREMENTS    OF   THE   EARTH'S    DIAMETER. 


Polar  diameter 
Equatorial  diameter 
Compression     -    - 


7899.17 

7925.64 

26.47 


Bessel. 


7899.11 

79i5.60 

26.49 


293 


APPENDIX. 


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QUESTIONS    FOR    CLASS  USE. 


These  are  the  questions  which  the  author  has  used  in  his  own  classes 
for  review  and  examination.  In  the  historical  portion,  he  has  required 
his  pupils  to  write  articles  upon  the  character  and  life  of  the  various 
persons  named,  gathering  materials  from  every  attainable  source.  He 
has  also  introduced  whatever  problems  the  class  could  master,  taking 
topics  from  the  article  on  Celestial  Measurements  and  the  various 
mathematical  treatises. 

Introduction. — Define  Astronomy.  Is  the  earth  a  planet?  Is  the 
moon  a  planet?  What  is  the  sky?  Why  does  it  seem  concave  ?  What 
gives  it  its  color?  What  is  the  difference  in  the  appearance  of  a  fixed 
star  and  a  planet?  What  is  the  Milky  Way?  In  what  direction  does 
it  span  the  heavens?     In  what  season  of  the  year  is  it  most  brilliant  ? 

I.    THE    HISTORY. 

5-6.  What  can  you  say  of  the  antiquity  of  astronomy  ?  How  far 
back  do  the  Chinese  records  extend  ?  Name  some  astronomical  phe- 
nomena they  contain.  Why  were  these  astronomers  at  fault  in  failing 
to  announce  the  eclipse  ?  (Ans.  Certain  religious  ceremonies  were 
performed  on  such  occasions,  and  their  omission  was  believed  to  expose 
the  nation  to  the  anger  of  the  gods.)  Why  should  the  Chaldeans  have 
become  versed  in  this  study  ?  How  ancient  are  their  records  ?  What 
discoveries  did  they  make?  How  does  the  Asiatic  differ  from  the  Euro- 
pean mind  ? 

7.  What  Grecian  philosopher  early  acquired  a  reputation  in  thi? 
science?  What  other  discovery  did  Thales  make  (Physics,  p.  251)? 
What  did  he  teach?  What  memorable  eclipse  did  he  predict?  What 
did  Anaximander  teach?  In  what  century  did  Pythagoras  live?  What 
was  his  characteristic  trait  ?  Did  he  advance  any  proof  of  his  system  ? 
Explain  his  theory.  How  does  it  differ  from  ours?  What  strange 
views  did  he  hold  ? 

8.  When  did  Anaxagoras  live?  What  did  he  teach?  What  theory 
did  Eudoxus  advance?    What  is  the  theory  of  the  cr3'stalline  spheres? 


294  QUESTIONS  FOR  CLASS   USE. 

What  has  Hipparchus  been  styled  ?  What  addition  did  he  make  to 
astronomical  knowledge  ?  How  man}-  stars  in  our  present  catalogue 
(p.  207;?  How  did  Egypt  rank  in  science  at  an  early  day?  What 
preparation  did  the  Grecian  philosophers  make  to  fit  themselves  for 
teachers?     How  long  did  Pythagoras  travel  for  'his  purpose? 

9.  What  can  you  say  of  the  School  at  Alexandria  ?  What  great  work 
did  Ptolemy  write?  What  theory  did  he  expound?  Was  it  original? 
What  discovery  did  Eratosthenes  make  ?  Describe  that  method  (p.  282). 
Show  how  the  movements  of  the  planets  puzzled  the  ancients. 

10.  What  was  the  theory  of  "  cycles  and  epicycles "  ?  Did  the 
ancients  believe  in  the  reality  of  this  cumbrous  machinery?  Did  this 
theory  possess  anj*  accuracy  ? 

11.  Could  it  be  adapted  to  explain  any  new  motion  ?  What  was  the 
remark  of  Alfonso  ?  Describe  the  progress  of  learning  among  the 
Saracens. 

12.  Where  was  the  first  Observatorj'  in  Europe  built?  W^hen  did 
Spain  lose  her  prominence  in  scientific  studies? 

13.  What  is  astrology  ?  What  was  its  association  with  astronomy? 
State  something  of  the  repute  in  which  astrolog)-  was  held.  Tell  what 
you  can  of  the  system.     W'hat  use  did  it  subserve  ? 

14.  What  theory  displaced  the  Ptolemaic?  W'hen?  Was  the  system 
of  Copernicus  original  ?  What  credit  is  due  him?  Describe  his  idea 
of  apparent  motion.  How  did  he  apply  this  to  the  heavenly  bodies  ? 
What  crudity  did  he  retain  ? 

15.  Who  was  Tycho  Brahe  ?  What  was  his  theory?  How  did  it 
differ  from  Ptolemy's  and  Copernicus's  ?  What  good  did  Brahe 
accomplish?  Could  he  generalize  his  facts?  Had  he  a  telescope  ? 
How  did  Kepler  differ  from  Brahe?  What  were  the  two  prominent 
characteristics  of  Kepler  ? 

16-19.  State  his  three  laws.  Tell  how  he  discovered  the  first.  The 
«econd.  The  third.  Describe  (he  ellipse.  Define  focus,  perihelion, 
and  aphelion.  What  remarkable  statement  did  Kepler  make?  When 
did  Galileo  live  ? 

20.  What  discoveries  did  he  make  in  Physics?  In  astronomy? 
WTiat  advantage  did  he  have  over  his  predecessors?  Give  an  account 
of  his  observations  on  the  moon.     On  Jupiter's  moons. 

21.  Why  did  this  settle  the  controversy  between  the  Ptolemaic  and 
the  Copernican  system?  How  were  Galileo's  discoveries  received? 
Give  some  of  Sizzi's  arguments.  Who  discovered  the  law  of  gravita- 
tion ? 

23.  Repeat  it.  How  was  this  idea  suggested  ?  WTiat  familiar  laws 
of  motion  aided  Newton?  How  did  he  apply  these  to  the  motion  ot 
the  moon?     Repeat  the  storj-  of  his  patient  triumph. 


THE   SOLAR  SYSTEM.  295 

24,  25.  What  is  the  celestial  sphere  ?  Give  the  two  illustrations  which 
show  its  vast  distance  from  the  earth.  Why  can  we  not  see  the  stars 
by  day,  as  by  night?  What  portion  of  the  sphere  is  visible  to  us? 
Name  the  three  systems  of  circles. 

26-30.  Name  and  define  (i)  the  principal  circle,  (2)  the  secondary 
circles,  (3)  the  points,  and  (4)  the  measurements  of  each  system.  De- 
fine especially,  because  in  common  use,  zenith,  nadir,  azimuth,  alti- 
tude, equinoctial,  right  ascension,  declination,  equinox,  ecliptic,  colure, 
and   solstice.     What  is  N  or  S  in  the  heavens  ? 

31.  What  is  the  Zodiac?  How  wide  is  it?  How  ancient?  How 
is  it  divided  ?  State  the  names  and  signs.*  State  the  meaning  of  each 
(p.  210.) 

II.    THE    SOLAR    SYSTEM. 

What  bodies  compose  the  solar  system  ?  Describe  how  we  are  t& 
picture  it  to  ourselves. 

The  Sun. — What  is  its  sign?  Its  distance  from  us?  Illustrate. 
What  is  the  solar  parallax  (see  pp.  121,  275)?  What  change  has  recentl>' 
been  made  in  the  estimate  of  the  parallax  of  the  sun,  and  of  its  distance 
from  the  earth?    (See  p.  279.) 

37.  How  are  celestial  distances  measured?  What  is  the  color  of  the 
sun  ?     To  what  is  the  sun's  light  equal  ?     To  how  many  moons  ? 

38.  To  what  is  its  heat  equal?  Illustrate.  What  proportion  of  the 
sun's  heat  reaches  the  earth?  What  is  the  apparent  size  of  the  sun? 
How  does  this  vary? 

39.  40.  State  the  solar  dimensions,  (i)  diameter— illustrate  ;  (2) 
volume  ;  (3)  mass  ;  (4)  weight  ;  (5)  density.  How  large  did  Pythagoras 
think  the  sun  is?  Tell  something  about  the  force  of  gravity  on  the 
sun.  How  much  would  you  weigh  if  carried  to  its  surface?  (The  force 
of  gravity  on  the  sun  as  compared  with  the  earth  is  27.6.)  How  does 
the  sun  appear  to  the  naked  ej'e? 

41.  How  can  we  seethe  spots?  What  were  formerly  the  views  of 
astronomers  with  regard  to  the  sun's  face  ? 

42.  When  were  the  spots  discovered?  Tell  something  about  the 
number  of  the  spots.  Their  location."  Size.  What  number  of  miles 
subtend  a  second  of  arc  at  the  distance  of  the  sun? 

*  "The  Ram,  the  Bull,  the  Heavenly  Twins, 
And  ne.\t  the  Crab,  tlie  Lion  shines, 

The  Virgin  and  the  Scales, 
The  Scorpion,  .Archer,  and  He-goat, 
The  Man  that  bears  the  watering-pot, 

And  Fish  with  glittering  tail?," 


296  QUESTIONS   FOR   CLASS   USE. 

43.  Describe  the  parts  of  which  the  spots  are  composed.  Describe 
the  motion  of  the  spots. 

44.  How  do  the  spots  change  in  form  as  they  pass  across  the  disk? 
What  does  this  prove  ?     What  is  the  length  of  a  solar  axial  rotation  ? 

45.  46.  Explain  a  sidereal  and  a  synodic  revolution  of  a  spot.  Why 
do  not  the  spots  move  in  straight  lines?  Show  how  they  curve.  Tell 
what  you  can  about  the  irregular  movements  of  the  spots. 

47.  Illustrate  how  suddenly  they  change.  What  can  j-ou  say  about 
their  periodicity?  Who  discovered  this?  Is  there  any  connection  be- 
tween the  solar  spots  and  the  aurora  ? 

48.  Tell  about  the  influence  of  the  planets  on  the  spots.  Do  the 
spots  aflfect  the  truitfulness  of  the  season  ?  Does  the  temperature  of  the 
spots  diflfer  from  that  of  the  rest  of  the  sun  ?  Are  the  spots  depressions 
in  the  sun  ? 

49.  How  much  darker  are  they  than  the  adjacent  surface?  Is  the 
sun  brighter  than  the  Drummond  light?  {Ans.  "The  sun  gives  out 
as  much  light  as  one  hundred  and  forty-six  lime-lights  would  do,  if 
each  were  as  large  as  the  sun  and  were  burning  all  over.'') 

50.  What  are  the  faculae  ?  Describe  the  mottled  appearance  of  the 
sun.  What  is  the  shape  of  the  bright  masses?  What  is  a  granule? 
What  is  its  size  ? 

51.  52.  Describe  the  constitution  of  the  sun  according  to  Wilson's 
theor\'.  How  are  the  spots  produced  ?  The  faculse  ?  The  penumbra  ? 
The  nucleus  ?     The  umbra  ? 

53,54.  What  is  the  present  theor)' ("  KirchhofTs  Theory  ")  ?  Name 
the  four  different  portions  of  the  sun.  Define  the  nucleus.  The  photo- 
sphere. The  chromosphere.  The  corona.  What  are  the  protuber- 
ances? How  are  the  spots  produced  ?  The  umbra?  The  penumbra? 
Upon  what  discoveries  does  this  theory  depend  (p.  262)?  What  is  the 
cause  of  the  heat  of  the  sun  ?     Will  the  heat  ever  cease  ?* 

The  Planets. — Name  the  six  characteristics  common  to  all  the 
planets.     Compare  the  two  groups  of  the  major  planets. 

57,  58.  Draw  an  ellipse,  and  name  the  various  parts.  Define  the 
ecliptic.  The  plane  of  the  ecliptic.  Why  is  the  ecliptic  so  called? 
Define  the  ascending  node.  The  descending  node.  Line  of  the 
nodes.  Longitude  of  the  node.  Tell  what  )'ou  can  with  regard  to 
the  comparative  size  of  the  planets. 

*  If  we  accept  the  Nebular  hypothesis  (p.  256),  we  must  suppose  that  the  heat  is 
produced  by  the  condensation  of  the  nebulous  matter  and  consequent  chemical 
changes.  The  sun  is  radiating  its  heat  constantly,  and,  at  some  time,  its  light  will  go 
out,  in  turn,  as  that  of  the  earth  and  the  planets  has  before  it.  This  theory  is  of  especial 
mterest,  as  it  shows  that  the  sun,  as  well  as  the  solar  system,  has  a  certain  fixed  exist- 
ence; and  that,  "  like  all  natural  objects,  it  passes  through  its  regular  stages  of  birtt). 
vigor,  decay,  and  death,  in  ope  order  of  progress." — Newcomb, 


THE  SOLAR  SYSTEM.  W 

60.  What  is  a  conjunction  ?     Name  the  earliest  that  are  recorded. 

61-3.  What  do  you  say  concerning  the  probability  of  the  planets  being 
inhabited  ?*  State  the  conditions  of  life  on  the  different  planets.  What 
are  the  two  divisions  of  the  planets? 

64.  What  causes  the  apparently  irregular  movements  of  the  planets  ? 
Define  heliocentric  and  geocentric  places.  Illustrate.  In  what  part 
of  the  sky  is  an  inferior  planet  always  seen?  Define  inferior  and 
superior  conjunction.     Greatest  elongation. 

65.  Why  is  a  star  atone  time  "  evening  "  and,  at  another,  "  morning 
star  "  ?     What  is  a  transit  ^ 

65,  66.  Explain  the  retrograde  motion  of  an  inferior  planet.  (This 
motion,  it  will  be  remembered,  was  one  that  sorely  puzzled  the 
ancients.)     Describe  the  phases  of  an  inferior  planet. 

67.  Why  does  an  inferior  planet  have  phases  ?  Define  gibbous. 
Explain  the  opposition  and  conjunction  of  a  superior  planet. 

68.  Explain  its  retrograde  motion.  Must  a  superior  planet  always 
be  seen  in  the  same  part  of  the  sky  as  the  sun?  Define  quadrature. 
Can  an  inferior  planet  be  in  quadrature? 

69.  70.  Which  retrogrades  more,  a  near  or  a  distant  planet?  Define 
a  sidereal  and  a  synodic  revolution  of  an  inferior  and  a  superior  planet, 
and  tell  what  you  can  about  each.  In  what  case  would  there  be  no 
difference  between  a  sidereal  and  a  synodic  revolution  ?  Why  is  a  planet 
invisible  when  in  conjunction?  When  is  a  planet  evening,  and  when 
morning  star  ? 

71.  Tell  what  you  can  about  the  supposed  discovery  of  a  planet  in- 
terior to  Mercury.     What  name  and  sign  have  been  given  to  it? 

Mercury. — Definition  and  sign?  Describe  the  appearance  of  Mer- 
cury, and  where  seen.  What  was  the  opinion  of  the  ancients  ?  Of  the 
astrologists  ?  Of  chemists  ?  Why  is  it  difllicult  to  see  this  planet  ? 
When  can  we  see  it  best  ? 

73.  What  is  the  peculiarity  of  its  orbit?  What  is  Mercury's  greatest 
elongation  from  the  sun?  Why  does  this  vary?  What  is  Mercury's 
distance  from  the  sun?     What  is  its  velocity?     What  is  the  length  of 

*  The  uniformity  of  Nature  is  a  most  effective  argument  in  this  direction.  Light 
travels  everywhere  through  the  universe  at  the  same  rate.  The  elements  of  star 
planet,  and  the  earth  are  the  same.  The  sun,  which  may  be  considered  as  the  mother 
of  the  earth,  is  composed  of  the  same  materials.  The  laws  of  gravitation  rule  so 
absolutely  that  the  satellite  ofSiriuswas  not  discovered  until  after  it  was  observed 
that  an  unknown  influence  afiected  the  star.  "  The  uniformity  of  law  and  mattei  is 
proof  that  there  must  be  through  the  universe  organizations  similar  to  those  of  our 
system.  We  see  the  result  of  these  laws  in  the  world  we  inhabit,  and  we  cannot 
doubt  that  the  same  powers  and  the  same  materials  have  produced  organizations 
similar  to  these  of  the  earth  in  millions  of  other  places,  though  we  can  only 
philosophically  suppose  their  existence,  not  practically  prove  it." — IV.  Meytr, 


298  QUESTIONS   FOR  CLASS  USE. 

its  day?     Of  its  year?     What  is  the  difference  between  its  sidereal  and 
synodic  revolutions  ?     What  is  its  distance  from  the  earth? 

74-6.  Show  why  its  greatest  and  least  distances  vary  so  much.  What 
is  its  diameter?  Volume?  Density?  Force  of  gravity?  Specific 
gravity?  How  much  would  you  weigh  on  Mercury?  (Mercury's  foice 
of  gravity  as  compared  with  that  of  the  earth  is  46.)  Describe  its  sea- 
sons. (If  the  pupil  does  not  understand  pretty  well  the  subject  of  the 
terrestrial  seasons,  it  would  be  well  here  to  read  carefully  page  95, 
et  seq.)  What  is  the  temperature?  The  appearance  of  the  sun  ?  Has 
Mercury  any  moon?  What  is  the  appearance  of  the  planet  through  a 
telescope?  What  do  these  phases  prove?  What  do  we  know  of  the 
mountains  and  valleys  upon  Mercurj'?  The  atmosphere?  Have  we 
any  recent  observations  ? 

77.  Venus.* — Definition  and  sign?  Ancient  names?  Appearance 
to  us?     When  brightest ?     Can  Venus  be  seen  by  daj-?     Illustrate. 

78.  Describe  the  orbit  of  Venus.  What  is  the  distance  of  Venus 
from  the  sun?  Velocity?  Length  of  the  year?  Day?  Difference 
between  the  sidereal  and  synodic  revolutions?  Distance  from  the 
earth? 

79.  How  near  may  Venus  approach  us?  How  does  her  apparent 
size  vary?  When  is  Venus  the  brightest?  What  is  her  diameter? 
Volume?     Density? 

So.  Force  of  gravity  ?  (The  force  of  gravity  on  the  surface  of  Venus 
is  .82  that  of  the  earth.)  Docs  the  force  of  gravity  increase  or  decrease 
with  the  mass  or  volume  of  the  body?  Describe  the  seasons  upon 
Venus. 

81,82.  Describe  the  telescopic  appearance  of  Venus.  Who  dis- 
covered the  phases  of  Venus  ?  What  was  the  effect  of  this  discovery? 
What  proof  have  we  that  Venus  possesses  a  dense  atmosphere?  Has 
Venus  a  moon  ? 

83.  E.\RTH. — Sign  ?  What  is  the  appearance  of  the  earth  from  the 
other  planets?  Do  we,  then,  live  on  a  star?  Is  it  probable  that  the 
earth  was  always  dark  and  dull  as  it  now  seems  to  us?*     How  does 

♦  Venus  Is  the  only  planet  mentioned  by  Homer — 

O'os  S'a.<rTr\p  elcri    ncr'  dcTTpaffi  wKxdi  a/xoXyw 
*E(J7r€po5  b?  KdAAio"To?  €v  ovpavio  itTTaTcu.  atTTrjp 

Iliad^  xxii.  317. 

*  Probably  not.  The  earth  was  doubtless  once  a  glowing  star,  like  the  sun.  Its 
crust  is  only  the  ashes  and  cinders  of  that  fearful  contJagration.  The  rocks  are  all 
burnt  bodies.  The  atmosphere  is  only  the  gas  left  over  after  the  fuel  was  all  con- 
sumed. Every  organic  object  has  been  rescued  by  plants  and  the  sunbeam  from  the 
grasp  u!  oxygen. 


The  solar  system.  299 

the  size  of  the  earth  compare  with  that  of  the  other  planets?  What  is 
the  shape  of  the  earth  ?  What  is  its  exact  diameter  ?  (See  Table  in 
the  Appendix.) 

84.  Circumference?  Density?  Weight?  What  comparison  may  be 
made  to  illustrate  its  inequalities?  How  do  you  prove  the  rotundity 
of  the  earth  ?  * 

85.  Why  can  we  see  further  from  the  top  of  a  hill  than  from  its  base  ? 
Why  is  the  horizon  a  circle  ? 

86-7.  Give  some  illustrations  of  apparent  motion.  Is  it,  then,  natu- 
ral for  us  to  transfer  motion  ?  Under  what  conditions  do  you  think 
this  occurs  ?  Explain  the  cause  of  the  rising  and  setting  of  the  sun 
and  stars.  Who  first  explained  these  phenomena  in  this  manner? 
What  do  you  say  of  its  simplicity  ? 

88.  What  is  the  cause  of  day  and  night?  Do  all  places  on  the 
earth  revolve  with  equal  velocity  ?  Illustrate.  At  what  rate  do  we 
move?     Wh}'  do  we  not  perceive  our  motion? 

89.  What  would  be  the  effect  if  the  earth  were  to  stop  its  rotation? 
Is  there  any  danger  of  this  catastrophe?  How  is  the  length  of  the  day 
increasing?     Is  the  amount  appreciable  ? 

90-1.  Draw  the  figure,  and  show  how  the  stars  move  daily  through 
unequal  orbits  and  with  unequal  velocities.  Describe  the  appearance 
of  the  stars  at  the  N.  Pole.     At  the  Equator.     At  the  S.  Pole. 

92-3.  Describe  the  path  of  the  earth  about  the  sun.  Defin  eccentri- 
cit}\  What  is  the  amount  of  the  eccentricity  of  the  earth's  orbit?  Is 
this  stable?  Do  we  see  the  same  stars  at  different  seasons  of  the  year  ? 
Why  not?  If  we  should  watch  from  6  p.  M.  to  6  .k.  m.,  what  portion  of 
the  sphere  would  we  see? 

94.  What  do  we  mean  by  the  yearly  motion  of  the  sun  among  the 
stars  ?  How  can  we  see  it  ?  What  is  the  cause  ?  What  is  the  ecliptic  ? 
Why  so  called  ?  What  are  the  equinoxes?  What  do  you  understand 
when  you  see  in  the  almanac  the  statement  that  *'  The  earth  is  in 
Aries?"  "  The  sun  is  in  Sagittarius?"  etc.  How  many  apparent 
motions  has  the  sun  ?  Name  them,  and  give  the  cause  and  effects  of 
each.     Has  the  sun  any  real  motions? 

95.  Describe  the  apparent  motion  of  th  sun,  N.  and  S.  How  is  it 
that  the  sun  in  summer  shines  on  the  north  side  of  some  houses  both 
at  rising  and  setting,  but  in  winter  never  does?  Define  the  obliquity 
of  the  ecliptic.  The  parallelism  of  the  earth's  axis.  What  do  you  say 
of  its  permanence  ?  Why  will  a  top  stand  while  spinning,  but  will  fall 
as  soon  as  it  ceases  ? 

97.  Show  how  the  rays  of  the  sun  strike  the  various  parts  of  the 

*  It  is  said  that  aeronauts,  at  a  proper  height,  can  distinctly  see  the  curving  form 
of  the  earth's  sui£ice. 


300  QUESTIONS  FOR  CLASS  USE. 

earth  at  different  angles  at  the  same  time.  Show  how  the  angles  vary 
at  different  times.  Is  the  sun  reall}'  hotter  in  summer  than  in  winter? 
Why  does  it  seem  to  be  ?  Why  is  it  warmer  in  summer  than  in  win- 
ter ?  What  effect  upon  the  temperature  has  the  difference  in  the  length 
of  the  summer  and  the  winter  day? 

98-100.  Explain  the  cause  of  equal  day  and  night  at  the  equinoxes. 
\Vh)' are  our  days  and  nights  of  unequal  length  at  all  other  times? 
Why  does  the  length  vary  at  different  seasons  of  the  year  ?  How  do 
the  seasons,  &c.,  in  the  N.  Temperate  Zone  compare  with  those  in  the 
S.  Temperate  Zone?  Describe  the  yearly  path  of  the  earth  about  the 
sun — (i),  at  the  summer  solstice  ;  (2),  at  the  autumnal  equinox  ;  (3),  at 
the  winter  solstice  ;  (4),  at  the  vernal  equinox  ;  (5),  the  yearly  path 
finished  back  to  the  starting-point.  Is  the  division  of  the  earth's 
surface  into  zones  an  artificial  or  a  natural  distinction?  Who  in- 
vented it? 

loi.  How  much  nearer  are  we  to  the  sun  in  winter  than  in  summer? 
Why  is  it  not  warmer  in  winter?  How  is  it  in  the  South  Temperate 
Zone?  W^hen  do  the  extremes  of  heat  and  cold  occur?  Why  do  they 
not  occur  exactly  at  the  solstices? 

102.  Why  is  summer  longer  than  winter  ?  Does  the  earth  m  ve 
with  the  same  velocity  in  all  parts  of  its  orbit?  Describe  the  curious 
appearance  of  the  sun  at  the  North  Pole.  In  Greenland,  at  what  part 
of  the  year  will  the  midnight  sun  be  seen  due  north  ?  What  is  the 
length  of  the  days  and  nights  at  the  Equator? 

103.  Describe  the  results  if  the  axis  of  the  earth  were  perpendicular 
to  the  ecliptic.     If  the  equator  were  perpendicular  to  the  ecliptic. 

104-5.  Define  the  precession  of  the  equinoxes.  Who  discovered 
this  fact  ?  At  what  rate  does  this  movement  proceed  ?  What  time  will 
be  required  for  the  equinoxes  to  make  an  entire  revolution  ?  What 
are  the  results  of  precession?  What  star  was  formerly  the  Polar  Star? 
(See  p.  219.) 

106-9.  Explain  the  cause  of  precession.  How  does  the  spinning  of  a 
top  illustrate  ihis  subject  ?  In  what  way  is  the  force  which  acts  on  a 
spinning-top  opposite  10  that  which  produces  precession  ?  What  is 
Nutation  ?  What  is  the  cause  of  the  nodding  motion  ?  How  does  the 
moon's  influence  compare  with  that  of  the  sun  ?  W'hat  is  the  effect  of 
Nutation? 

no.  What  is  the  real  path  of  the  N.  Pole  through  the  hea%'ens  ?  Is 
the  obliquity  of  the  ecliptic  invariable  ?  What  is  the  period  of  this 
oscillation  ? 

III.  What  causes  combine  to  produce  this  nodding  motion  we  have 
described  ?  Why  are  the  tropics  located  where  they  are  ?  Is  their 
position  on  the  earth  permanent?     What  effect  does  precession  have 


THE  SOLAR  SYSTEM.  301 

on  the  latitude  of  the  stars  ?  What  is  meant  by  the  line  of  apsides  of 
the  earth's  orbit?  Is  this  permanent ?*  What  is  the  Great  Year  of 
the  Astronomers  ? 

112.  At  what  season  of  the  year  is  the  earth  now  in  perihelion  ? 
When  was  it  in  perihelion  in  the  autumn?  When  :n  the  winter? 
When  will  perihelion  occur  in  the  spring?  When  in  summer? 
When  will  the  cycle  be  completed  ?  What  provision  is  there  for  per- 
manence in  the  midst  of  these  changes? 

113-14.  What  is  refraction?  Its  effect?  Upon  what  principle  of 
Optics  is  this  based  ?  How  does  refraction  vary  ?  Are  the  sun  and 
moon  ever  where  they  seem  to  be  ?  Is  the  real  day  longer  or  shorter 
than  the  apparent  one  ? 

115.  Describe  the  apparent  deformation  of  the  sun  and  moon  near 
the  horizon.  Explain.  Why  are  not  these  bodies  apparently  deformed 
in  the  same  way  when  they  are  high  m  the  heavens  ?  Why  do  they 
appear  smaller  in  the  latter  case  ?  (See  Fig.  48,  p.  124.)  What  causes 
the  hazy  appearance  of  the  heavenly  bodies  near  the  horizon  ? 

116.  What  is  the  cause  of  twilight  ?  How  long  does  it  last?  Is  it 
the  same  at  all  seasons  of  the  year?     In  all  pans  of  the  earth  ? 

117.  Where  is  it  the  longest?  Shortest?  State  the  cause  of  this 
variation.  What  is  diffused  light?  What  would  be  the  effect  if  the 
atmosphere  did  not  act  in  this  way?  Is  there  really  any  sky  in  the 
heavens  ?  What  is  the  cause  of  the  appearance  ?  What  is  aberration 
of  light  ? 

118.  Illustrate  this  phenomenon.  State  two  reasons  why  we  never 
see  the  sun  where  it  really  is. 

119.  What  is  the  general  effect  of  aberration  ?  Define  parallax.  Il- 
lustrate. 

120-21.  Define  true  and  apparent  place.  How  does  parallax  vary  ? 
What  is  the  practical  importance  of  this  subject  (pp.  36,  278)?  Define 
horizontal  parallax.  What  is  the  sun's  horizontal  parallax  ?  What  is 
the  annual  parallax? 

The  Moon. — Signs  ?  Describe  the  moon's  orbit.  What  is  the 
moon's  distance  from  the  earth?  Illustrate.  What  is  the  difference 
between  her  sidereal  and  synodic  revolutions  ?  What  is  the  real  path 
ot  the  moon?  (Imagine  a  pencil  fastened  to  the  spoke  of  a  wheel,  and 
the  wheel  rolled  by  the  side  of  a  wall  on  which  the  pencil  is  constantly 
marking.)     How  often  does  the  moon  turn  on  her  axis  ?     What  is  the 

*  "The  line  of  equinoxes  of  the  earth's  orbit,  as  we  have  seen,  has  a  slow  left- 
handed  retrograde  motion  of  so".2  eacli  year,  called  the  precession  of  the  equinoxes  \ 
and  the  line  of  apsides  has  a  still  sXowftx  right-handed  direct  motion  oi  \\".2<)\  and 
in  consequence  of  the  motion  of  both  these  lines,  the  an^le  formed  by  them  changes 
through  6i".49  each  year,  so  as  to  complete  an  entire  revolution  in  21,077  years." 


302  QtJESTlONS  FOR  CLASS  USE. 

moon's    diameter?    Volume?     How  does   her    apparent    size  vary? 
Why  does  she  appear  larger  than  she  really  is  ? 

124.  Why  does  the  crescent  moon  appear  larger  than  the  dark  body 
of  the  moon?  When  ought  the  moon  to  appear  the  largest?  Do  all 
persons  think  the  moon  to  be  of  the  same  apparent  size? 

125.  Explain  the  three  librations  of  the  moon.  How  does  moonlight 
compare  with  sunlight?     Is  there  any  heat  in  moonlight? 

126.  Does  the  center  of  gravity  in  the  moon  coincide  with  that  of 
magnitude  ?  Has  the  moon  any  atmosphere  ?  What  proof  have  we  of 
this?  (Ans.  (i).  We  see  but  slight,  if  any,  appearance  of  twilight  on 
the  moon.  (2).  When  the  moon  passes  between  us  and  a  star,  it  does 
not  refract  the  light  of  a  star,  so  that  the  atmosphere  cannot  be  suflS- 
cient  to  support  more  than  y§ij  of  an  inch  of  the  mercurial  column.) 
What  must  be  the  effect  of  this  lack  upon  the  temperature  of  the  moon's 
surface?  State  Langley's  observations  upon  Mount  Whitney.  How 
does  the  earth  appear  from  the  moon? 

127-9.  What  is  the  earth-shine  ?  How  is  it  caused  ?  What  is  it  called 
in  England  ?  Describe  the  path  of  the  moon  around  the  earth,  and  the  con- 
sequent phases.  Why  is  new  moon  seen  in  the  west  and  full  moon  in  the 
east  ?  Why  can  we  sometimes  see  the  moon  in  the  west  after  the  sun  rises, 
and  in  the  east  before  the  sun  sets?  What  is  the  length  of  a  lunar 
month?*  What  do  we  mean  by  the  moon's  running  high  or  low? 
What  is  the  cause  of  this  variation?     Is  it  of  any  use? 

130-1.   What  is  harvest  moon  ?     What  is  the  cause  ?f 

132.  Explain  the  cause  of  "Dry  Moon"  and  "Wet  Moon."  What 
are  nodes?  How  much  is  the  moon's  orbit  inclined  to  the  ecliptic — • 
our  ideal  sea-level?  What  is  an  occultaiion?  What  use  does  it  sub- 
serve?    Describe  the  seasons,  heat,  etc.,  on  the  moon. 

135-7.  Describe  the  telescopic  appearance  of  the  moon.  Are  the 
mountains  the  light  or  the  dark  portions?  What  canyon  say  about 
them?  The  gray  plains?  The  rills?  The  craters?  What  are  the  pecu- 
liar features  of  the  lunar  landscapes?    Are  the  lunar  volcanoes  extinct? 

*  "  The  moon's  sidereal  period  is  not  constant,  and  a  comparison  of  modern  with 
ancient  observations  shows  that  it  has  undergone  an  acceleration  since  tlie  period  ot" 
the  Chaldean  observations  of  eclipses  made  720  b.  c.  Several  explanations  have  been 
given  by  Laplace  and  others,  of  the  supposed  cause  of  the  acceleration  of  the  moon's 
mean  motion  ;  but  it  is  highly  probable  that  it  is  a  pseudo-phenomenon^  that  owes  its 
origin  to  a  real  lengthening  of  the  time  of  rotation  of  the  earth  (which  is  the  unit 
of  astronomical  time),  caused  by  the  friction  of  the  sea  and  atmosphere." 

+  It  will  aid  in  understanding  the  cause  of  harvest  moon,  if  one  gets  clearly  in 
mind  the  fact  that  the  moon  when  full  is  always  in  the  opposite  part  of  the  heavens 
from  the  sun.  At  the  time  of  the  autumnal  equinox,  i.  e.  when  the  sun  is  at  the 
autumnal  equinox,  (or  in  Libra,  note,  p.  94.)  the  moon  must  be  at  the  vernal  equinox, 
(or  in  Aries.)  The  least  retardation  of  the  moon,  which  occurs  at  this  time,  happens, 
therefore,  in  September. 


SOLAJEl  SYSTEM.  303 

138.  Eclipses. — When  can  an  eclipse  of  the  sun  occur  ?  Show  how 
a  solar  eclipse  may  be  total,  partial,  or  annular.  Define  umbra.  Pe- 
numbra. Central  eclipse.  State  the  general  principles  of  a  solar 
eclipse.  What  curious  phenomena  attend  a  total  eclipse?*  What 
are  Daily's  Beads  ?  What  is  the  corona  ?  Describe  the  effect  of  a  total 
eclipse.  What  curious  custom  prevails  among  the  Hindoos?  What 
is  the  Saros?  Is  it  now  of  any  value?  What  is  the  metonic  cycle? 
Explain  its  use.  What  is  the  golden  number?  What  is  the  cause  of  a 
lunar  eclipse  ?  Why  are  lunar  eclipses  seen  oftener  than  solar  ones? 
How  were  total  eclipses  formerly  regarded  ? 

147.  The  Tides. f — Define  ebb.  Flow.  How  often  does  the  tide 
happen?  Explain  the  cause.  Why  does  the  tide  occur  about  fifty 
minutes  later  each  day?  Why  is  there  a  tide  on  the  side  opposite  the 
moon?  The  sun  is  much  larger  than  the  moon  ;  why  does  it  not  pro 
duce  the  larger  tide?  What  effect  has  the  friction  of  the  tides  produced 
upon  the  earth?  What  theory  upon  this  topic  has  Professor  Ball 
advanced  ?  What  is  meant  by  the  differential  effect  of  tlie  moon  ?  Why 
is  not  the  tide  felt  out  at  sea?  What  is  spring-tide  ?  Neap-tide?  Why 
does  the  tide  differ  so  much  in  various  localities?  Tell  about  the 
height  of  the  tides  at  different  points.  Why  is  there  no  tide  on  a  lake  ? 
Is  the  tidal  wave  a  forward  movement  of  the  water  ? 

150.  Mars. — Definition  and  sign?  Describe  the  appearance  of  this 
planet.  When  is  it  brightest?  What  is  its  distance  from  the  sun? 
Velocity?  Day?  Year?  Distance  from  the  earth?  What  is  the  pecu- 
liarity of  its  orbit?  What  is  the  diameter  of  Mars?  Its  volume  and 
density  as  compared  with  the  earth?  How  far  would  a  stone  fall  on 
its  surface  the  first  second?  Who  discovered  its  moons  ?  What  is  the 
peculiarity  of  these  tiny  globes?  What  are  the  peculiar  telescopic 
features  of  Mars?  What  is  the  cause  of  its  ruddy  color?  Wliat  are 
the  snow-zones?    Can  we  watch  the  change  of  its  seasons? 

*  Lockyer,  describing  the  beginning  of  a  total  eclipse,  says :  "  One  seems  in  a  new 
world— a  world  filled  with  awful  sights  and  strange  forebodings,  and  in  which  still- 
ness and  sadness  reign  supreme ;  the  voice  of  man  and  the  cries  of  animals  are  hushed ; 
the  clouds  are  full  of  threatenings  and  put  on  unearthly  hues  ;  dusky,  livid,  or  purple, 
or  yellowish  crimson  tones  chase  each  other  over  the  sky  irrespective  of  the  clouds. 
The  very  sea  is  responsive  and  turns  lurid  red.  All  at  once  the  moon's  shadow  comes 
sweeping  over  air,  and  earth,  and  sky,  with  frightful  speed.  Men  look  at  each  other 
and  behold,  as  it  were,  corpses,  and  tlie  sun's  light  is  lost."— Gillis.  in  his  observa- 
tions upon  the  eclipse  of  1859,  as  witnessed  by  him  m  Peru,  remarks:  "At  1.54,  the 
moment  of  totality,  the  attendants,  catching  sight  of  the  corona,  dropped  on  theii 
knees,  and  shouted,  '•  La  Gloria !  La  Gloria ! " 

t  As  the  tidal  wave  does  not  move  as  rapidly  as  the  earth  does,  the  water  has  an 
apparent  backward  motion.  It  has  been  suggested  that  this  (as  well  as  the  friction 
of  the  atmosphere)  acts  as  a  break  on  the  earth's  diurnal  revolution.  It  has  been 
shown  that  the  moon's  true  place  can  be  best  calculated  if  we  suppo-e  that  the  side- 
real day  is  shortening  at  the  rate  of  iHU  of  a  second  in  2,400  years.  (See  page  89.) 


304  QUESTIONS  FOR  CLASS  USE. 

154.  Minor  Planets  (Asteroids).* — Give  Bode's  law.  Telthow  the 
first  of  these  planets  was  discovered.  How  many  are  now  known? 
Are  the)-  probably  all  discovered  ?  Describe  some  of  these  "  pocket 
planets".  Do  they  all  lie  within  the  Zodiac?  What  is  their  origin ? 
{Ans.  According  to  the  nebular  hypothesis,  the  ring  of  matter  broke 
up  into  numberless  small  bodies,  instead  of  aggregating  into  one  large 
planet.)     Give  some  of  the  names  and  signs. 

157.  Jupiter. — Definition  and  sign?  Describe  his  appearance.  De- 
scribe his  orbit,  \yhai  is  his  distance  from  the  sun?  Velocity?  Day? 
Year?  Distance  from  the  earth?  Diameter?  Volume?  Density? 
Centrifugal  force ?  Force  of  gravity  ?  Figure?  Describe  his  seasons. 
Upon  what  does  the  change  of  seasons  in  any  planet  depend?  What 
must  be  the  appearance  of  the  Jovian  sky?  Describe  the  telescopic 
features  of  Jupiter.  Are  Jupiter's  moons  visible  to  the  naked  eye? 
What  are  their  names?  What  is  their  size?  What  space  do  they 
occupv?  Describe  the  eclipse  of  Jupiter's  moons.  Define  immersion, 
emersion,  and  transit.  How  rapidly  do  the  satellites  revolve?  What 
can  you  say  of  the  frequency  of  eclipses  on  Jupiter?  Describe  the  belts. 
Why  are  they  parallel  to  its  equator?  How  was  the  velocity  of  light 
discovered?  Does  Jupiter  emit  light?  Is  it  probable  that  a  solid 
crust  has  formed  over  this  planet?  In  what  waj-  is  Jupiter  repro- 
ducing the  earth's  histori"? 

164.  Saturn. — Definition  and  sign  ?  Describe  Saturn's  appearance 
in  the  heavens.  How  nipidly  does  this  planet  move  through  the  sky? 
What  is  its  distance  from  the  sun  ?  What  is  the  peculiarity  of  its 
orbit  ?  What  is  its  velocity  ?  Year  ?  Day  ?  Distance  from  the  earth  ? 
Diameter?  Volume?  Density?  Force  of  gravity  ?  Describe  its  sea- 
sons. Has  it  any  atmosphere  ?  Who  discovered  the  rings  of  Saturn  ? 
Describe  them.  Which  are  the  Bright  Rings  ?  Which  is  the  Dusky 
Ring?  Are  they  stationary?  E.\plaiu  their  phases.  Of  what  are 
they  composed?  Does  Saturn  emit  !ight?  Describe  Saturn's  belts. 
Describe  Saturn's  moons.     The  scenerj'  on  Saturn. 

170.  Ur.\nus. — Definition  and  sign  ?  How  was  this  planet  dis- 
covered ?  Tell  of  its  previous  observation  by  Le  Monier.  Is  Uranus 
visible  to  the  naked  eye  ?     What  is  its  distance  from  the  sun  ?     Year  ? 

*  "  It  may  surprise  some  persons  to  learn  that  the  total  mass  of  the  two  or  three 
hundred  small  planets  which  have  been  discovered  between  the  orbits  of  .Mars  and 
Jupiter,  is  sufficient  only  to  make  a  globe  a  little  over  403  miles  in  diameter.  In  other 
words  if  our  globe  were  divided  into  8,000  equal  parts,  one  of  these  parts  would  equal 
in  bulk  and  in  weight  the  toUl  of  all  these  asteroids.  Or,  cut  the  earth  through  the 
equator,  then  take  a  section  of  about  three-fourths  cf  a  mile  in  thickness,  and  it  would 
furnish  material  for  all  these  small  planets  and  something  remaining.  It  would  seem 
that  the  solar  system  could' not  be  much  damaged,  if  some  of  these  small  planets 
should  drop  out  of  their  courses  and  join  some  of  the  larger  ones." 


THE  SOLAR  SYSTEM,  305 

Diameter?     Density?     Describe    its    seasons.      Telescopic    features. 
Satellites.     Peculiarity  of  its  moons. 

172.  Neptune.— Definition  and  sign?  What  is  the  appearance  of 
this  planet  in  the  sky  ?  Give  an  account  of  its  wonderful  discovery. 
What  is  its  distance  from  the  sun?  Year?  Velocity?  Diameter? 
Volume  ?  Density  ?  Do  we  know  anything  of  its  seasons  ?  Why  not  ? 
What  is  the  appearance  of  the  heavens?  What  are  the  telescopic 
features  of  Neptune?  Has  Neptune  any  moon?  What  advantage 
have  the  Neptunian  astronomers  ? 

175.  Meteors,  Aerolites,  and  Shooting-Stars.— Define  an  aero- 
lite. A  shooting-star.  A  meteor.  Give  some  account  of  the  fall  of 
aerolites.  What  elements  are  found  in  aerolites?  How  can  an  aero- 
lite be  distinguished?  Give  an  account  of  wonderful  meteors.  Of 
shooting-stars. 

176.  Describe  the  showers  of  1799  and  1833.*  The  shower  of  1866.  At 
what  intervals  did  these  showers  occur  ?  Why  was  not  the  shower  of 
1866  seen  in  this  country  ?  (Ans.  Our  side  of  the  earth  was  not  turned 
toward  the  meteors.)  What  is  the  average  number  of  meteors  and 
shooting  stars  daily?  Why  do  we  not  see  more  of  them?  In  what 
months  are  they  most  abundant  ?f  Describe  the  origin  of  meteors  and 
shooting-stars.  What  is  their  velocity?  What  causes  the  light?  The 
explosion  often  heard  ?  What  is  the  theory  of  meteoric  rings?  What 
is  their  shape  ?  How  do  these  streams  of  meteoroids  account  for  the 
showers  at  regular  intervals?  What  is  the  period  of  the  November 
ring?  Why  is  the  August  shower  so  uniform,  while  the  November 
one  is  periodic  ?:J:     What  is  the  relation  between  meteors  and  comets? 

*  A  southern  planter,  describing  the  effect  of  the  star-shower  of  1833,  says:  ''I 
was  suddenly  awakened  by  the  most  distressing  cries  that  ever  feU  on  my  ears. 
Shrieks  of  horror  and  cries  for  mercy  I  could  hear  from  most  of  the  negroes  of  three 
plantations,  amounting  in  all  to  about  600  or  800.  While  earnestly  listening  for  the 
cause,  I  heard  a  faint  voice  near  my  door  calling  my  name.  I  arose,  and  taking  my 
sword,  stood  at  the  door.  At  this  moment  I  heard  the  same  voice  still  beseeching  me 
to  rise,  and  saying,  '  Oh,  my  God,  the  world  is  on  fire  !'  I  then  opened  the  door, 
and  it  is  difficult  to  say  which  excited  me  most,  the  awfulness  of  the  scene  or  the 
cries  of  the  distressed  negroes.  Upwards  of  one  hundred  lay  prostrate  on  the  ground, 
some  speechless,  and  some  with  the  bitterest  cries,  with  their  hands  raised,  implor- 
ing God  to  save  the  world  and  them.  The  scene  was  truly  awful,  for  never  did  rain 
fall  much  thicker  than  the  meteors  towards  the  earth :  east,  west,  north,  and  south,  it 
was  the  same." 

+  It  has  been  noticed,  from  very  early  times,  that  the  night  of  the  10th  of  August 
(St.  Laurence's  Day)  is  especially  favorable  ff)rthe  occurrence  of  shooting-stars;  and 
in  Catholic  Ireland,  these  stars,  on  the  toth  of  August,  are  always  called  the  "  tears 
of  St.  Laurence  the  Martyr,"  who  was  put  to  death  by  being  broiled  upon  a  gridiron. 

t  The  fact  that  the  November  meteoroids  are  collected  in  a  shoal  instead  of  be- 
ing distributed  uniformly  through  the  orbit  gives  color  to  the  idea  that  this  stream 
has  not  been  long  a  member  of  the  solar  systcip,    "  In  1867,  Leverrier  stated  his  be- 


306  QUESTIONS  FOR   CLASS   USE. 

What  do  3-ou  mean  by  the  radiant  point  ?  What  is  the  height  of 
meteors  ?     Weight  ? 

185.  Comets. — How  were  comets  looked  upon  by  the  ancients?  Il- 
lustrate.    Define  the  term  comet.     What  is  the  tail?*     The  nucleus? 

lief  that  the  November  meteors  form  the  remains  of  some  comet  that  had  been  re- 
cently introduced  into  the  solar  system  by  the  attraction  of  one  of  the  large  outer 
planets.  He  lound  that  the  year  a.  d.  126  would  give  a  position  to  the  planet  Uranus 
capable  of  producing  such  an  eflFect,  by  converting  the  parabolic  path  of  a  comet  into 
the  path  now  described  by  the  November  meteors.  In  the  year  a.  d.  137,  the  changed 
path  of  the  comet  for  the  first  time  came  near  the  earth  in  her  orbit  round  the  Sun, 
since  which  year  the  petrified  comet  or  shower  of  stones  has  completed  52  entire  re- 
volutions, the  last  of  which  terminated  on  the  13th  of  November,  1866.  Theophanes 
of  Byzantium  relates  that  in  November,  a.  d.  472,  the  sky  at  Constantinople  appeared 
to  be  on  Are  with  flying  meteors.  This  corresponded  with  the  tenth  revolution  of 
the  November  meteors. — Conde,  in  his  history  of  the  dominion  of  the  Arabs,  speak- 
ing of  the  year  a.  d.  902,  states  that  in  the  month  of  October  (13th),  on  the  night  of 
the  death  of  King  Ibrahim  Ben  Ahmed,  an  immense  number  of  falling  stars  were 
seen  to  spread  themselves  over  the  face  of  the  sky  like  rain,  and  tliat  the  year  in  ques- 
tion was  thenceforth  called  the  '  Year  of  Stars'  This  year  corresponded  to  the 
twenty-third  revolution  of  the  November  meteors. — A  similar  shower  of  stars  took 
place  on  the  17th  of  October,  a.  d.  934. — On  the  14th  of  October,  a.  d.  1002,  a  remark- 
able shower  of  shooting-stars  is  noted  by  the  Arab  astronomers  and  historians,  cor- 
responding with  the  completion  of  the  twenty-sixth  revolution  of  the  November 
meteors. — It  is  related  in  the  annals  of  Cairo  that  on  the  19th  of  October,  a.  d.  1202, 
the  stars  appeared  like  waves  upon  the  sky,  towards  the  east  and  west ;  they  flew 
about  like  grasshoppers,  and  were  dispersed  from  left  to  right.  This  shower  corres- 
ponded with  the  thirt3'-second  revolution  of  the  November  meteors.— On  the  22nd  of 
October,  a.  d  1366,  a  shower  of  stars  was  noted,  corresponding  with  the  thirty- 
seventh  revolution  of  the  November  meteors.— A  similar  phenomenon  (forty-second 
revolution)  was  observed  on  the  25th  of  October,  a.  d.  1533. — The  forty-seventh 
revolution  was  noted  on  the  9th  of  November,  a.  d.  1698. — The  fiftieth  revolution, 
observed  by  Humboldt  and  Boupland,  on  the  12th  of  November,  a.  d.  1799,  as  already 
remarked,  first  led  modern  astronomers  to  speculate  on  the  true  nature  of  these  re- 
markable periodic  phenomena. — The  early  observations  of  this  meteoric  shower  were 
dated  on  the  12th  of  October,  and  during  52  revolutions  the  intersection  of  its  orbit 
with  that  of  the  earth  has  moved  on  to  the  14th  of  November. — Mr.  Adams  hasshown 
this  movement  of  nodes  to  be  a  consequence  of  the  attractions  of  the  superior  planets, 
and  has  finally  demonstrated  the  truth  of  the  cometary  origin  of  the  November 
meteors."' — Houghton. 

*  "  Comets  are  almost  alwaj-s  accompanied  by  tails,  which  are  placed  in  the  line 
joining  the  Sun  and  Comet,  and  on  the  side  opposite  to  the  Sun.  Exceptions  to  this 
rule,  though  rare,  sometimes  occur.  For  e.xample.  the  tail  of  the  Comet  of  1577  de- 
viated 21  °  from  the  line  joining  the  Sun  and  the  Comet,  and  the  tail  of  the  Comet  of 
1680  diverged  5^  from  the  same  line.  Comets  have  been  occasionally  observed  with 
two  tails,  one  in  the  usual  position,  and  the  other  in  nearly  an  opposite  direction,  or 
towards  the  Sun.  The  angle  between  the  two  tails,  when  such  a  phenomenon  has 
been  observed,  has  always  been  very  considerable,  varying  from  140°  to  170°. 
This  rare  phenomenon  of  two  tails  is  supposed  to  be  connected  with  certain  rapid 
changes  which  the  gaseous  substance  of  the  Comet  is  obser\'ed  to  undergo  on  ap- 
proaching the  Sun.  There  are  many  instances  on  record,  in  which  the  tails  of  Comets 
were  observed  to  stretch  through  100°  of  the  celestial  sphere,   and  the  apparent 


THE   SIDEREAL   SYSTEM.  307 

The  head?  The  coma?  Does  each  comet  necessarily  possess  all 
these  parts  ?  How  would  a  mere  round,  fleecy  mass  be  known  to  be 
a  comet  ?  What  mistake  did  Herschel  make  in  looking,  as  he  sup- 
posed, at  one  of  this  kind  (p.  171)?  Where  do  comets  appear?  In 
vvhat  direction  do  they  move?  How  does  a  comet  look  when  first 
seen?  Describe  the  approach  of  a  comet  to  the  sun.  Upon  what  does 
the  time  of  greatest  brilliancy  depend  ?  What  do  you  say  of  the  num- 
ber of  the  comets  ?  What  was  Kepler's  remark  ?  Why  do  we  not  see 
them  oftener?  Where  did  Lockyer  see  one?  Describe  the  orbits  of 
comets.  Which  class  has  been  calculated  ?  Which  classes  never  re- 
turn? Describe  the  difficulty  of  calculating  a  comet's  orbit.  Name 
the  periods  of  some  comets.  What  has  been  the  distance  from  the  sun 
of  some  noted  comets  ?  Velocity  ?  What  do  you  say  of  the  density  of  a 
comet?  Illustrate.  Is  there  any  danger  of  our  running  against  a 
comet  ?  Do  comets  shine  by  their  own  or  by  reflected  light  ?  Tell  what 
you  can  of  their  variation  in  form  and  dimensions.  Give  some  account 
of  the  comets  of  1811,  1835,  ^"d  1843.  For  what  is  Biela's  comet 
noted  ?  (Ans.  "  A  very  remarkable  phenomenon  attended  the  perihelion 
passage  of  this  comet  in  the  latter  end  of  1845.  It  became  divided 
into  two  comets,  which  did  not  again  re-unite,  but  traveled  along  to- 
gether in  similar  orbits.  This  unique  phenomenon  was  noticed  for 
the  first  time  in  America  on  the  29th  of  December.  The  greatest  dis- 
tance observed  between  these  two  fragments  of  Biela's  comet,  before 
their  final  disappearance,  was  about  iivo-thirds  of  the  moon's  distance 
from  the  earth.")  For  what  is  Encke's  comet  noted  ?  What  is  its  period  ? 
Give  some  description  of  Donati's  comet.     The  comet  of  1882. 

196.  Zodiacal  Light. — Wherecan  this  be  seen  ?    What  is  its  appear- 
ance?    Where  is  it  brightest?     What  is  its  origin  ? 


III.    THE     SIDEREAL    SYSTEM. 

203.  Tell  something  of  the  appearance  of  the  heavens  at  Neptune's 
distance  from  the  sun — our  starting-point.  Do  we  ever  see  the  stars? 
What  do  we  see,  then  ?  Whicli  star  is  nearest  the  earth  ?  What  is  its 
parallax?  Its  distance?  How  long  would  it  take  light  to  reach  the 
nearest  star?  How  would  the  earth's  orbit  appear  at  that  distance  ?  Our 
sun?  How  long  does  it  take  for  the  light  of  the  smaller  stars  to  reach 
the  earth?  What  can  you  say  of  the  motion  of  the  fixed  stars?    Illustrate. 

length  of  the  tail  is  known  to  undergo  most  rapid  changes.  We  shall  mention  only 
one  case  as  an  example  of  this  phenomenon.  The  Comet  of  1618  presented  to  the 
Danish  astronomer,  Longomontanus,  a  tail  of  104°  in  length,  while  it  had  been 
measured  by  Kepler  a  few  days  previous,  and  ascertained  to  be  only  70°  long," 


308  QUESTIONS  FOR  CLASS  USE. 

What  proof  have  we  that  the  fixed  stars  are  suns  ?*  Describe  the  motion 
of  the  solar  system.  Is  the  center  known  ?  How  many  stars  can 
we  see  with  the  naked  eye  ?  With  a  telescope  ?  Have  all  the  stars 
been  discovered  ?  What  is  the  cause  of  the  twinkling  of  the  stars? 
Do  we  know  anything  of  the  magnitude  of  the  stars  ?  Name  the  points 
of  difference  between  a  planet  and  a  fixed  star.  What  do  you  mean 
by  a  star  of  the  first  magnitude?  How  many  are  there  ?  Of  the  second 
magnitude  ?  How  many  sizes  can  one  see  with  the  naked  eye  ?  What  is 
the  cause  of  the  difference  in  the  brightness  ?  How  are  the  stars  named  ? 
Describe  the  division  of  the  stars  into  constellations.  Is  there  any  real 
likeness  to  the  mythological  figures?  Name  any  figure  which  seems 
to  you  well  founded.  Are  the  boundaries  distinct?  Who  invented 
the  system  ?  State  the  possible  meaning  of  the  signs  of  the  Zodiac  and 
their  origin.  Explain  why  the  signs  and  the  constellations  of  the 
Zodiac  do  not  agree.  What  causes  the  appearance  of  the  constella- 
tions? Would  they  appear  as  they  now  do,  if  we  should  go  out  into 
space  among  them?  Are  the  present  forms  permanent?  State  the  value 
of  the  stars  in  practical  life.  What  were  the  views  of  the  ancients  with 
regard  to  the  stars?  Describe  the  division  of  the  stars  into  three  zones. 
214.  The  Constell.\tions. — The  questions  on  these  are  uniform  : 
( i)  description.  {i\  principal  stars,  and  (3)  mythological  history.  Therefore, 
they  need  not  be  repeated  with  each  constellation.  What  are  the 
pointers?  Does  Polaris  mark  the  exact  position  of  the  North  Pole? 
How  man}- times  per  day  is  Polaris  on  the  meridian  of  any  place? 
Explain  how  this  applies  in  navigation  or  surveying.  State  how  the 
amount  of  the  variation  from  the  true  north  will  change  through  the 
ages.  What  star  will  ultimately  become  the  pole-star?  What  curious 
facts  are  stated  concerning  the  Pyramids?  What  do  you  say  ot  the  dis- 
tance of  Polaris  ?  How  razy  latitude  be  calculated  by  means  of  Polaris  ? 
What  stars  never  set  in  our  sky?    What  stars  never  rise?f     Will  the 

*  Sinus  shines  at  least  200  times  as  brightly  as  our  sun  would  shine  if  set  beside 
it.  Assuming  its  surface  to  be  equally  brilliant,  this  would  imply,  in  comparison 
with  our  sun,  a  diameter  14  times  and  a  volume  3.000  times  as  great.  Its  luster, 
however,  seems  higher  than  the  sun's,  but,  even  making  allowance  for  that,  we  must 
stUl  consider  this  giant  sun  to  be  at  least  1,000  times  as  large  as  our  own  orb.  Re- 
cent evidence  tends  to  show  that  its  rate  of  recession  from  us  is  diminishing,  so  that 
we  may  e.xpect  this  to  change  into  a  motion  of  approach.  Here  is  a  hint  that  Sirius 
is  travelling  in  a  mighty  orbit  with  movements  carrying  it  alternately  from  and  toward 
us — Proctor. 

+  All  stars  whose  north  polar  distance  is  less  than  the  latitude  of  any  place,  will 
never  set  at  that  place,  and  all  stars  whose  south  polar  distance  is  less  than  the  latitude, 
will  never  rise.    The  Greeks  and  the  Romans  were  familiar  with   the  fact  that  cer- 
tain stars  never  descend  below  the  horizon.    The  following  quotations  are  interesting  : 
'•  Immunemque  aequoris  Arcton."' 

Ovid,  Met.\m.  xiii.    293. 
■•  Arctos 
^quoris  expertes."  Jo.  726—7. 


THE   SIDEREAL  SYSTEM.  309 

Big  Dipper  always  appear  as  now  ?  Name  three  bright  stars  near  the 
first  meridian.  (Ans.  a  Andromedse,  >■  Pegasi,  and  ,i  CassiopeiiE.)  How 
many  degrees  of  longitude  correspond  to  an  hour  of  time?  At  what 
rate  is  Sirius  receding  from  the  earth?  How  has  this  motion  been  dis- 
covered?    (See  page  261.) 

239.  Double  Stars,  etc. — Does  any  star  appear  double  to  the  naked 
eye  ?  How  many  have  been  found  by  the  use  of  the  telescope  ?  What 
is  an  optical  double  star?  Are  all  double  stars  of  this  class  ?  Describe 
the  revolution  of  a  binary  system.  What  other  combinations  have  been 
discovered?  What  are  their  periods?  Orbits?  Mass?  Are  these 
companion  stars  as  close  to  each  other  as  they  seem  ? 

241.  Name  some  prominent  colored  stars.  Do  their  colors  ever 
change?  Which  colors  would  indicate  the  hottest  star?  What  is  the 
probable  effect  in  a  system  having  colored  suns  ? 

242.  What  are  variable  stars  ?  Describe  the  changes  of  Algol.  Of 
Mira.     What  is  the  cause  ? 

243.  What  are  temporary  stars?  Describe  the  one  seen  in  Cassio- 
peia. The  one  in  Corona  Borealis,  in  1866  ?  What  are  lost  stars  ? 
Can  you  give  any  explanation  of  this  phenomenon?  Of  what  did  the 
star  of  1866  consist?  Are  these  stars  destroyed?  Is  the  process  of 
creation  now  complete  ? 

245.  What  are  star  clusters  ?  Name  several.  Is  such  a  grouping  a 
mere  optical  effect  ?  Are  they  probably  as  closely  compacted  as  they 
seem  to  be  ? 

246.  What  are  nebulse  ?  How  do  they  differ  from  clusters?  Is  it 
probable  that  all  nebulse  will  be  resolved  into  clusters  ?  What  is  the 
general  belief  concerning  nebulne  ?  What  has  spectrum  analysis  proved 
some  of  the  nebulae  to  be  ?  Where  are  they  most  abundant  ?  What 
can  you  say  about  their  distances?  Into  how  many  classes,  for  con- 
venience, are  they  divided  ?  Describe  and  illustrate  the  elliptic  nebu- 
lae.    What  is  said  of  the  distance  of  the  great  nebula  in  Andromeda? 

'ApKTOi'  9'  rjv  KoX  aixa^av  iTriK\ri<nv  KoXfovatv, 
'Ht'  avTOv  <TTpe<f)eTai,  Kai  t   'fipiwra  SoKeiiii, 
OtT)  S'  d/iifiopos  e(7Ti  Xoerpuiv  'fiKeoroio, 

Iliad,  xviii.  487—9.  and  Odys.  v.  273—5. 

*ApKTOt  Kvav^ov  Tre^uAa-yjuterot  *n*cearoio. 

Aratus.  Ph.«nom.  48. 
"  Arctos  oceani  metuentes  jequore  tingui." 

ViRG.  Georg.  L  246. 
In  order  to  understand  the  meaning  of  the  expressions  7re<t)v\ayix€i'oi  'flKeat'oio,  and 
^^  aquoris  expertes"  as  used  by  a  Greek  or  Italian,  we  should  remember  that  the 
north  polar  distance  ofr/  Ursse  Majoris  1839°  56'  48"  ;  and  since  the  latitude  of  Athens 
is  37"  58',  and  that  of  Naples  40°  50',  an  inhabitant  of  the  former  city  would  see  this 
star  descend  below  the  northern  horizon  for  a  small  portion  of  its  course ;  and  an 
inhabitant  of  Naples  would  see  it  sink  within  3'  of  the  horizon,  so  as  just  to  move 
along  its  northern  edge. 


310  QUESTIONS  FOR   CLASS  USE. 

The  number  of  stars  it  contains  ?  Describe  the  annular  nebulae.  What 
is  said  of  the  "  ring  universe  "  in  Lyra?  Its  diameter?  Describe  the 
spiral  nebula  in  Canes  Venatici.  Describe  the  planetar>'  nebulae. 
What  is  said  of  the  number  and  size  of  these  "island  universes"? 
Describe  the  fantastic  appearance  of  the  irregular  nebulae. 

251.  What  are  nebulous  stars?  What  is  their  structure?  What 
are  variable  nebulae  ?  Give  instances.  What  is  said  of  double 
nebulae  ?  Is  an)-thing  definite  known  with  regard  to  them  ?  What 
are  the  Magellanic  clouds  ? 

253.  Describe  the  Milky-way.  What  can  you  say  of  the  number  of 
stars  in  the  Galaxy?     Are  the  stars  uniformly  distributed  ? 

254.  What  is  Herschel's  theory  of  the  constitution  of  the  universe? 
If  this  theory  be  true,  what  is  our  sun  ? 

255.  Give  an  account  of  the  Nebular  hypothesis.  What  is  said  of 
Saturn's  rings  ?     May  they  ultimately  disappear? 

259.  What  is  spectrum  analysis?  Name  the  three  kinds  of  spectra. 
What  colored  rays  will  a  flame  absorb?*  Describe  the  spectroscope. 
What  are  Fraunhofer's  lines?  What  is  known  of  the  constitution 
of  the  sun  ?  What  proof  have  we  that  iron  exists  in  the  sun  ?  What 
elements  have  been  found  in  the  sun  ?     What  proof  have  we  that  the 


*  The  power  which  gases  possess  of  cutting  out  the  particular  lines  which  belong 
to  the  color  that  each  emits  has  been  beautifully  illustrated  by  Prof.  Newcomb.  He 
says:  "■  Suppose  nature  should  loan  us  an  immense  collection  of  many  millions  of 
gold  pieces,  out  of  which  we  were  to  select  those  which  would  ser\'e  us  for  money, 
and  leturn  her  the  remainder.  The  English  rummage  through  the  pile,  and  pick  out 
all  the  pieces  which  are  of  the  proper  weight  for  sovereigns  and  half-sovereigns  ;  the 
French  pick  out  those  which  will  make  five,  ten,  twenty,  or  fifty-franc  pieces;  the 
Americans  the  one,  five,  ten  and  twenty  dollar  pieces,  and  so  on.  After  all  the  suit- 
able pieces  are  thus  selected,  let  the  remaining  mass  be  spread  out  on  the  ground 
according  to  the  respective  weights  of  the  pieces,  the  smallest  pieces  being  placed  in 
a  row,  the  next  in  weight  in  an  adjoining  row,  and  so  on.  We  shall  then  find  a  num- 
ber of  rows  missing:  one  which  the  French  have  taken  out  for  five-franc  pieces; 
close  to  it  another  which  the  Americans  have  taken  for  dollars  ;  afterwards  a  row 
which  have  gone  for  half-sovereigns,  and  so  on.  By  thus  arranging  the  pieces,  one 
would  be  able  to  tell  what  nations  had  culled  over  the  pile,  if  he  only  knew  of  what 
weight  each  one  made  its  coins.  The  gaps  in  the  places  where  the  sovereigns  and 
half-sovereigns  belonged  would  indicate  the  English,  that  in  the  dollars  and  eagles 
the  Americans,  and  so  on.  If,  now,  we  reflect  how  utterly  hopeless  it  would  appear, 
from  the  mere  examination  of  the  miscellaneous  pile  of  pieces  which  had  been  left,  to 
ascertain  what  people  had  been  selecting  coins  from  it,  and  how  easy  the  problem 
would  appear  when  once  some  genius  should  make  the  proposed  arrangement  of  the 
pieces  in  rows,  we  shall  see  in  what  the  fundamental  idea  of  spectrum  analysis  con- 
sists. The  formation  of  the  spectrum  is  the  separation  and  arrangement  of  the  light 
which  comes  from  an  object  on  the  same  system  by  which  we  have  supposed  the 
gold  pieces  to  be  arranged.  The  gaps  we  see  in  the  spectrum  tell  the  tale  of  the  at- 
mosphere through  which  the  light  has  passed  us;  in  the  case  of  the  coins  they  would 
tell  what  rutions  had  sorted  over  the  'p\\Qj"  —Nrwcomb's  Astronomy^  p.  228. 


THE   SIDEREAL  SYSTEM.  311 

stars  are  suns?  What  can  you  say  of  the  similarity  existing  between 
the  stars  and  our  earth  ?  What  has  been  discovered  with  regard  to 
the  constitution  of  the  Nebulae  ?  Of  their  relative  brightness?  How 
has  the  proper  motion  of  the  stars  been  shown? 

263.  Time. — What  two  methods  of  measuring  time?  What  is  a  si- 
dereal day?  What  are  astronomical  clocks?  Tell  how  they  are  used. 
Why  do  astronomers  use  sidereal  time  ?  What  is  a  solar  day?  What 
causes  the  difference  between  a  sidereal  and  a  solar  day  ?  To  how  much 
time  is  a  degree  of  space  equal?  Which  is  taken  as  the  unit,  the  solar 
or  the  sidereal  day?  How  long  is  a  solar  day?  A  sidereal  day?  A 
solar  day  equals  how  many  sidereal  hours  ?  A  sidereal  day  equals  how 
many  solar  hours?  Describe  mean  solar  time.  What  is  apparent 
noon?  Mean  noon?  The  equation  of  time  ?  When  is  this  greatest  ? 
When  least  ?  When  do  mean  and  apparent  time  coincide  ?  Can  a 
watch  keep  apparent  time?  How  may  apparent  time  be  kept?  How 
can  it  be  changed  into  mean  time  ?  Tell  how  to  erect  a  sundial.  When 
will  a  sidereal  and  a  mean  time  clock  coincide  ?  A  mean-time  clock 
and  the  sundial?  How  did  the  ancients  measure  time,  before  the 
invention  of  clocks  and  watches?"  State  the  two  reasons  wh}'  the 
solar  days  are  of  unequal  length.  What  is  the  civil  day  ?  Who  invented 
the  present  division  ?     Describe  the  customs  of  various  nations.  What 

*  "  The  ancients  used  clepsvdrje  and  siin-dials,  to  measure  time.  The  clepsydrae, 
in  its  simplest  form,  resembled  the  hour-glass,  water  being  used  instead  of  sand,  and 
the  flow  of  time  being  measured  by  the  flow  of  the  water.  After  the  era  of  Archi- 
medes, clepsydrae  of  the  most  el.iborate  construction  were  common  ;  but  while  they 
were  in  use,  the  days,  both  winter  and  summer,  were  divided  into  twelve  hours  from 
sunrise  to  sunset,  and  consequently  the  hours  in  winter  were  shorter  than  the  hours 
in  summer;  the  ciepsydra,  therefore,  was  almost  useless  except  for  measuring  inter- 
vals of  time,  unless  different  ones  were  employed  at  different  seasons  of  the  year. 
The  sun-dial  was  a  great  improvement  upon  the  clepsydrae  ;  but  at  night  and 
in  cloudy  weather  it  could  not  be  used,  of  course,  and  the  rising,  culmination,  and 
setting  of  the  various  constellations  were  the  only  means  available  for  roughly  telling 
the  lime  during  the  night.  Indeed,  Euripides,  who  lived  480-407  b.  c,  makes  the 
Chorus  in  one  of  his  tragedies  ask  the  time  in  this  form  : — 

'  What  is  the  star  now  passing  ?' 

and  the  answer  is  : — 

'  The  Pleiades  show  themselves  in  the  east ; 
The  Eagle  soars  in  the  summit  of  heaven.' 

It  is  also  on  record  that  as  late  as  a.  d.  1108  the  sacristan  of  the  Abbey  of  Cluny  con- 
sulted the  stars  when  he  wished  to  know  if  the  time  had  arrived  to  summon  the 
monks  to  their  midnight  prayers  ;  and  in  other  cases,  a  monk  remained  awake,  and 
to  measure  the  lapse  of  time  repeated  certain  psalms,  experience  havmg  taught  him 
in  the  day,  by  the  aid  of  the  sun-dial,  how  many  psalms  could  be  said  in  an  hour. 
When  the  proper  number  of  psalms  had  been  said,  the  monks  were  awakened."— 
Lockyer. 


312  QUESTIONS   FOR   CLASS  USE. 

is  the  origin  of  the  names  of  the  days?*  What  is  the  sidereal  \-ear? 
The  mean  solar  year?  What  causes  the  difference?  What  is  the 
anomalistic  year?  How  did  the  ancients  find  the  length  of  the  \'ear? 
What  error  did  they  make  ?  What  was  the  result  ?  Give  an  account 
of  the  Julian  calendar.  The  Gregorian  calendar.  What  is  the  mean- 
ing of  the  terms  O.  S.  and  N.  S.  ?f  What  country  now  uses  O.  S.  ? 
When  was  the  change  adopted  in  England  ?  X  How  was  it  received  ? 
How  could  a  child  be  eight  years  old  before  a  return  of  its  birthday? 
When  do  the  Jews  begin  their  year?  Why  does  our  year  begin  Jan- 
uary 1st  ?  Show  how  the  earth  is  our  timepiece.  What  influence  has 
Jupiter's  moons  on  the  cotton  trade? 

Celestial  Measlrements. — These  problems  are  to  be  used 
throughout  the  study.  They  require  no  questions  but  the  formal  state- 
ment of  the  problem  requiring  solution. 

*  It  is  said  that  the  Egyptians  named  the  seven  days  from  the  seven  celestial 
bodies  then  known.  The  order  was  continued  by  the  Romans  Tuesday  they  called 
Dies  Martis ;  Wednesday,  Dies  Mercurii :  Thursday,  Dies  Jovis ;  Friday,  Dies 
Veneris.  In  the  Saxon  mj-thology.  Tius.  Woden,  Thor,  and  Friga  are  equivalent  to 
Mars,  Mercury-.  Jupiter,  and  Venus.     Hence  we  see  the  origin  of  our  English  names. 

+  "  As  an  illustration  of  the  effect  of  the  change  of  style,  we  may  instance  the 
case  of  Washington.  He  was  bom  February  ii,  1732,  before  the  change  of  style. 
Inasmuch  as  1752  began  on  the  25th  of  March  and  ended  on  the  31st  of  December,  he 
had  no  birth-day  in  that  year  ;  hence,  he  was  20  years  old  on  the  22nd  of  February, 
1753,  new  style.  Because  anniversaries  are  always  determined  according  to  the 
civil  calendar,  the  birth-day  of  Washington  is  properly  celebrated  on  the  22nd  of 
February,  and  not  on  the  23d,  as  some  have  contended,  on  account  of  the  day  drop- 
ped in  the  year  1800." — Peck^s  Astronomy,  p.  216. 

X  "  In  England,  from  the  14th  century  till  the  change  of  style  in  1752.  the  legal  and 
the  ecclesiastical  year  began  March  25.  After  the  change  was  adopted  in  1752,  events 
which  had  occurred  in  January,  February,  and  before  March  of  the  old  legal  year, 
would,  according  to  the  new  arrangement,  be  reckoned  in  the  next  subsequent  year. 
Thus  the  revolution  of  1688  occurred  in  February  of  that  legal  year,  or.  as  we  should 
now  say,  in  February,  1689  ;  and  it  was,  at  one  time,  customary  to  write  the  date 
thus:  February,  i68|." — Appleton's  Cyclopaedia,  article  on  Calendar. 


GUIDE  TO  THE  CONSTELLATIONS. 


The  following  is  a  description  of  the  appearance  of  the  heavens  on  or  about  the 
first  da}'  of  each  month  in  the  year. 

January.  (7  p.  m.) — In  the  North,  Cassiopeia  and  Perseus  are 
above  Polaris,  Cepheus  and  Draco  west,  Ursa  Minor  fs  below,  and  Ursa 
Major  below  and  to  the  east.  ///  the  East,  Cancer  is  just  rising,  Canis 
Minor  (Procyon)  has  just  risen.  Along  the  Ecliptic,  Gemini  is  well  up, 
then  Taurus,  Aries — reaching  to  the  meridian,  next  Pisces;  Aquarius 
(letter  Y)  and  Capricornus  are  just  setting.  In  the  Southeast,  Orion  and 
the  Hare  are  well  up.  In  the  South,  Cetus  swims  his  huge  bulk  far  to  the 
east  and  west.  In  the  Southwest,  is  Piscis  Austral  is  (Fomalhaut).  North 
of  the  Ecliptic,  the  Triangles  are  nearly  in  the  zenith,  Perseus  is  just 
east,  below  is  Auriga,  Andromeda  lies  just  west  of  the  meridian,  and 
Pegasus  is  midway;  Delphinus  (the  Dolphin,  Job's  Coffin),  Aquila 
(Altair),  and  Lyra  (Vega)  are  fast  sinking  to  the  western  horizon  ;  while, 
along  the  Milky  Way,  blazes  the  brilliant  cross  of  Cygnus. 

February.  (7  p.  m.) — In  the  North,  Ursa  Major  lies  east  of  Polaris, 
Ursa  Minor  and  Draco  are  below,  Cepheus  is  west,  Cassiopeia  above 
and  to  the  west.  In  the  East,  Regulus  and  Cor  Hydrse  are  just  rising. 
Along  the  Ecliptic,  Leo  (Regulus,  the  sickle)  just  rising,  Cancer  well  up, 
Gemini  midway,  Taurus  on  the  meridian,  Aries  (the  scalene  triangle) 
past,  Pisces  next,  and,  lastly,  Aquarius  just  setting.  In  the  Southeast, 
Canis  Minor,  Canis  Major  ^Sirius),  and  Orion  are  conspicuous.  Iti  the 
Southivest,  Cetus  covers  nearly  the  whole  sky.  North  of  the  Ecliptic, 
Perseus  is  on  the  meridian,  while  Auriga  is  a  little  east  of  it ;  west  of 
Perseus  is  Andromeda,  while  the  Great  Square  of  Pegasus  is  fast 
approaching  the  horizon.     In  the  Northwest,  Cygnus  is  setting. 

March.  (7  p.  U.)—In  the  North,  Ursa  Major  lies  east  of  Polaris, 
Draco  and  Ursa  Minor  are  below,  Cepheus  is  below  and  to  the  west,  and 
Cassiopeia  west.  In  the  East,  Cor  Caroli  is  well  up,  toward  the  north- 
east, and  Coma  Berenices  is  rising.  Along  the  Ecliptic.  Leo  is  fully  risen, 
next  Cancer,  Gemini  reaches  to  the  meridian,  Taurus  is  past,  Aries 
midway,  and,  lastly,  Pisces  is  just  beginning  to  set.  In  the  Southeast,  Cor 
Hydrae,  Canis  Minor,  and  Canis  Major  are  conspicuous.   In  tfu  South, 


314  GUIDE  TO   THE  CONSTELLATIONS. 

Orion  blazes  brilliantl}'.  In  tlie  Southwest,  Cetus  is  hiding  below  the 
horizon.  Xorth  of  tJu  Ecliptic,  Auriga  is  in  the  zenith  ;  west  are  Per- 
seus and  Andromeda,  while  Pegasus  is  just  beginning  to  sink  out  of 
sight. 

April.  (7  p.  M.J — In  the  North,  Ursa  Major  is  above  and  to  the 
east  of  Polaris  ;  opposite  and  to  the  west  is  Perseus,  Draco  below  and 
to  the  east,  Cepheus  below  and  to  the  west,  Cassiopeia  west.  Jit  the 
East,  Bojtes  ( Arcturus)  is  not  quite  fully  r.sen.  Along  the  Ecliptic,  Virgo 
(Spica  is  rising,  Leo  midway,  Cancer  reaches  to  the  meridian,  Gemini 
is  past,  next  Taurus,  then  Aries,  and,  lastly.  Pisces  just  setting.  In 
the  Southeast,  is  the  Crater  (the  Cup) ;  Hydra  stretches  its  long  neck  to 
the  meridian.  In  tJu  South.  Canis  Minor.  In  tJie  Southioest,  Sirius  and 
Orion  ;  the  Eg^-ptian  X  ip.  229)  can  now  be  seen.  A'orth  of  t/ie Ecliptic, 
and  in  the  northeast,  are  Coma  Berenices  and  Cor  Caroli  ;  above 
Gemini  and  Taurus  is  Auriga,  while  Andromeda  is  just  setting  in  the 
northwest. 

3Ijiy.  (S  p.  M.) — In  the  Xorth.  Ursa  Major  is  above  Polaris,  Ursa 
Minor  and  Draco  are  east,  Cepheus  and  Cassiopeia  below,  and  Perseus 
is  west.  In  the  East,  Lyra  is  rising,  and  Hercules  is  just  up.  Along 
tlu  Ecliptic,  Libra  is  just  rising,  Virgo  is  midway.  Leo  is  on  the  me- 
ridian, Cancer  is  past,  next  Gemini,  and  lastly  Taurus  just  setting.  In 
t/i€  South,  stretching  east  and  west  of  the  meridian,  is  Hydra,  with  the 
Crater  and  Corvus  a  little  east.  /;/  tJte  South-u'cst,  are  Cor  Hydrae,  Canis 
Major,  and  Canis  Minor,  while  Orion  is  just  setting  in  the  west.  Noi-th 
of  the  Ecliptic,  in  the  east,  above  Hercules,  are  Corona  Borealis  (The 
Northern  Crown),  Bootes  (Arcturus),  Coma  Berenices,  and  Cor  Caroli, 
which  stretch  nearly  to  the  meridian.  In  tJie  Xorthzvest,  above  Taurus 
and  Perseus,  is  Auriga. 

Juue.  (S  p.  .M.I — /«  the  Xorth,  Ursa  Major  is  above  Polaris,  Draco 
and  Ursa  Minor  are  east,  Cepheus  is  below  and  east,  and  Cassiopeia 
directly  below.  ///  the  East,  Cj-gnus  (the  Cross)  and  Aquila  are  rising, 
Lyra  and  Taurus  Poniatowskii  are  well  up.  Along  the  Ecliptic,  Scoxp'xo 
is  rising.  Libra  is  midway,  Virgo  on  the  meridian,  Leo  past,  Cancer 
midway,  Gemini  next,  and  Taurus  just  setting.  In  the  South,  are  Cor 
vus  and  the  Crater  a  little  past  the  meridian.  In  tJie  Southwest,  is  Cor 
Hydrae,  and  in  the  west  Canis  Minor  is  nearing  the  horizon.  North 
of  the  Ecliptic,  in  the  east,  above  Scorpio,  is  Hercules  ;  then  Corona 
and  Bootes,  and,  near  the  meridian.  Cor  Caroli.  and  Coma  Berenices. 
In  the  Northwest,  is  Auriga,  just  coming  to  the  horizon. 

July,  (g  p.  M.) — In  the  Xorth,  Draco  and  Ursa  Minor  are  abo»e  Pola 
ris,  Ursa  Major  is  west,  Cepheus  east,  and  Cassiopeia  below  to  the  east. 
In  the  East,  the  Dolphin  (Job's  Coffin)  is  now  well  up,  Cygnus  is  almost 


GtrlDE  TO  THE  CONSTELLATIONS.  3l5 

midway  to  the  meridian,  and  Lyra  is  still  higher.  Along  the  Ecliptic, 
Capricornus  is  rising,  Sagittarius  (the  Archer)  is  next,  Scorpio,  with 
its  long  tail  swinging  along  the  horizon,  is  directly  south,  Libra  is  past 
the  meridian,  Virgo  midway,  and  Leo  has  almost  reached  the  horizon. 
In  the  South'west,  the  Crater  is  setting,  and  Corvus  is  just  above.  A'orth 
of  the  Ecliptic ,  above  Scorpio  and  east  of  the  meridian,  are  Serpentarius, 
Hercules,  and  Taurus  Poniatowskii  ;  Corona  is  almost  on  the  meridian, 
to  the  west  of  which  lie  Bootes,  Cor  Caroli,  and  Coma  Berenices. 

August.  (9  P.  M.) — In  the  North,  Draco  and  Ursa  Minor  are  above 
Polaris,  Cepheus  is  above  and  to  the  east,  Cassiopeia  east,  and  Ursa 
Major  west.  In  the  Northeast,  Perseus  is  just  rising,  while  south  of  it, 
Andromeda  and  Pegasus  are  fairly  up.  Along  the  Ecliptic,  Aquarius  is 
risen,  next  Caprjcornus,  Sagittarius  reaches  to  the  meridian.  Scorpio  is 
just  past,  Libra  next,  and  Virgo  (Spica)  just  touches  the  horizon.  North 
of  the  Ecliptic,  Taurus  Poniatowskii  is  on  and  Lyra  is  just  east  of  the 
meridian  ;  the  Swan  and  Dolphin  are  east  of  Lyra,  Serpentarius  and 
Hercules  are  above  Scorpio,  and  just  west  of  the  meridian  ;  thence  west 
are  Corona  and  Bootes,  while  far  in  the  northwest  are  Coma  Berenices 
and  Cor  Caroli. 

September.  (8  p.  m.) — Draco  is  above  and  to  the  west  of  Polaris, 
Cepheus  above  and  to  the  east,  Cassiopeia  east,  Ursa  Major  is  below 
and  to  the  west.  In  the  Northeast,  Perseus  is  just  rising.  In  the  East, 
Andromeda  is  fairly  up,  Pegasus  is  nearly  midway  to  the  meridian. 
Along  the  Ecliptic,  Pisces  is  just  rising,  next  Aquarius,  Capricornus  in 
the  southwest,  Sagittarius  on  the  meridian  in  the  south,  next  Scorpio  in 
the  southwest.  Libra,  and,  lastly,  Virgo  just  setting.  N^orth  of  the  Ecliptic, 
Lyra  is  on  the  meridian,  Cygnus,  the  Dolphin,  and  Aquila  are  just  to 
the  east ;  while  to  the  west,  are  Taurus  Poniatowskii  and  Serpentarius  ; 
north  of  these  latter  are  Hercules,  Corona,  Bootes,  Cor  Caroli,  and 
Coma  Berenices. 

October,  (7  P.  M.) — In  the  North,  Cepheus  and  Draco  are  above 
Polaris,  Ursa  Minor  is  west,  Cassiopeia  east,  and  Ursa  Major  below  and 
west.  In  the  Northeast,  Perseus  is  fairly  risen.  In  the  East,  An^xom- 
eda  is  nearly  midway  to  the  zenith.  Along  the  Ecliptic,  Aries  is  just 
rising,  Pisces  well  up,  Aquarius  and  Capricornus  are  in  the  southeast, 
Sagittarius  is  in  the  south,  Scorpio  far  down  in  the  southwest,  and  Libra 
just  setting.  North  of  the  Ecliptic,  Cygnus  and  Aquila  are  on  the  me- 
ridian ;  the  Dolphin  is  just  east  of  it,  and  far  south  ;  Lyra  is  west  of  the 
meridian  ;  Taurus  Poniatowskii  is  lower  down  and  to  the  south  ;  Ser- 
pentarius is  just  above  Scorpio  ;  next,  in  line  with  Scorpio  and  Polaris, 
is  Hercules ;  Corona  and  Bootes  are  toward  the  northwest,  where 
Coma  Berenices  is  just  setting. 


31  &  GUIDE   TO   THE    CONSTELLATIONS. 

November.  (7  p.  m.) — In  the  North,  Ursa  Major  is  below  Polaris, 
Ursa  Minor  and  Draco  are  to  the  west,  Cepheus  is  above,  and  Cassiopeia 
above  and  to  the  east.  In  the  Northecst,  Auriga  is  just  rising,  and 
Perseus  is  above,  nearly  midway  to  the  meridian.  Along  the  Ecliptic, 
Taurus  is  just  rising,  next  are  Aries  and  Pisces  ;  Aquarius  is  on  the  me- 
ridian, south  ;  then  Capricornus,  and  lastly  Sagittarius,  in  the  southwest. 
iVorth  of  the  Ecliptic,  Pegasus  and  Andromeda  lie  east  of  the  meridian, 
the  Swan,  Dolphin,  Eagle,  Taurus  Poniatowskii,  and  Lyra  west.  In 
the  Northwest,  are  Hercules  and  Corona. 

December.  (7  p.  m.) — In  the  North,  Cassiopeia  is  above  Polaris, 
Cepheus  above  and  to  the  west,  Perseus  above  and  to  the  east,  Draco 
west,  and  Ursa  Major  below.  In  tlie  NoriJieast,  below  Perseus,  is 
Auriga.  In  the  East,  Orion  is  rising.  Along  the  Ecliptic,  Gemini  is 
just  rising.  Taurus  is  nearly  midway,  next  Aries,  Pisces  is  on  the  me- 
ridian, then  Aquarius,  and  lastly  Capricornus,  far  in  the  southwest. 
/;/  the  South,  east  of  the  meridian,  is  Cetus,  and  west  is  Fomalhaut. 
A'orth  of  the  Ecliptic,  Andromeda  is  nearly  on  the  meridian,  and  Pega- 
sus west  of  it ;  Cvgnus,  Delphinus,  Lyra,  and  Aquila  are  about  midway, 
while  Taurus  Poniatowskii  is  just  sinking  10  the  horizon.  In  the  North- 
west, Hercules  is  just  setting. 

Note.— It  should  be  borne  in  mind  that  a  month  makes  a  variation  of  about  two 
hours  (30°)  in  the  rise  of  a  star  ;  hence,  in  the  foregoinp;  "  Guide,"  the  'January  Sky" 
of  9  p.  .M.  would  be  about  the  same  as  the  "  February  Sky  "  of  7  p.  m.  ;  the  "'January 
Sky"  of  II  p.  M.  would  be  about  the  same  as  the  "  March  Sky"  of  7  p.  m..  &c.  In 
this  way  the  "Guide"  may  be  used  for  any  hour  in  the  night.  The  pupil  will  see 
that  in  the  "  Guide  "  the  prominent  figures  and  stars  in  each  constellation  are  given 
in  parentheses.  Examples:  the  "  Y"  in  Aquarius,  the  -'scalene  triangle"  in  Aries, 
"Job's  Coffin  "  in  the  Dolphin,  "  Procyon  "  in  Canis  Minor,  &c.  These  aid  in  iden- 
tifying the  constellation. 


APPARATUS. 


To   ILLUSTRATE    PRECESSION 

OF  THE  Equinoxes,*  make  the 
simple  apparatus  shown  in  the 
cut.  It  represents  Fig.  38, 
and  the  explanation  of  that 
figure  and  several  subsequent 
ones  applies  to  it.  The  in- 
genuity of  pupil  and  teacher 
will  devise  methods  of  explain- 
ing by  means  of  this  instru- 
ment many  otherwise  abstruse 
points  under  this  difficult  sub- 
ject. The  following  are  sug- 
gestive merely  : 

1.  To  show  motion  of  earth' s 
axis  around  pole  of  Ecliptic. — 
Move  P,  axis  of  earth's  plate, 

around  D  F,  whose  circumference  roughly  represents  the  little  circle 
(ellipse)  described  by  the  pole  of  the  earth.     (Fig.  41.) 

2.  To  sho'o  change  of  Polar  Star. — The  pupil  can  readily  see  that  the 
north  pole  of  the  earth  will,  at  different  times,  point  to  different  stars 
located  around  this  circle.     Now,  Polaris;   next,  Lyra. 

3.  To  show  why  present  polar  distance  7oill gradually  diminish  and  then 
increase  {p.  217). — The  polar  star  lies  at  a  little  distance  from  this  circle 
(edge  of  plate)  and  the  pole  is  gradually  approaching  the  star,  but  will 
pass  it  and  then  recede  further  from  it,  until,  finally,  Lyra,  lyings"  from 
this  circle,  will  become  the  polar  constellation. 

4.  To  show  Precession  of  Equinoxes. —  Pass  axis  of  earth  around  small 

*  The  above  apparatus  was  devised  by  Solomon  Sias,  A.M.  M.D.,  Principal  of 
Schoharie  Union  School,  N.  Y.  It  can  be  made  by  any  ingenious  pupil.  The  plates 
are  cut  out  of  tin  ;  the  standard  may  be  made  with  the  knife  or  scroll-saw  to  suit 
one's  taste  ;  the  earth  is  half  of  a  little  wooden  ball  balanced  on  the  wire  pin  C  ;  and 
the  semicircle,  poles,  etc..  are  of  wire.  The  different  parts  may  be  soldered  or 
fastened  together  with  tacks. 


dl8  APPARATUS. 

circle.     Note  the  position  of  the  equinoxes  before  moving,  and  theif 
gradual  change  of  position  along  the  ecliptic. 

5.  To  show  cause  of  Precession. — Apply  explanation  of  Fig.  39. 

6.  To  show  necessity  f 01-  new  stellar  maps  occasionally,  or  careful  reduc- 
tions to  previous  standards.  With  the  change  of  equinoxes,  there  is 
also  a  change  of  the  equinoctial  system,  p.  27. 

7.  To  illustrate  Fig.  40. — Remove  the  wire  semicircle,  and,  inclining 
the  axis  of  the  earth,  spin  the  wire  between  the  thumb  and  finger  like 
a  top.  The  equinoxes  will  pass  around  the  ecliptic  as  they  did  when 
the  axis  was  carried  around  in  the  previous  experiments. 

Letting  G  B  E  represent  the  plane  of  the  ecliptic,  and  G  E  the  line 
of  the  equinoxes,  we  can  use  this  apparatus  to  illustrate  the  seasons, 
etc.,  (p  95).  Also,  by  placing  a  lamp  near  S,  the  phenomenon  of  day 
and  night,  long  summer  days,  short  winter  days,  etc.  (pp.  97,  etc.),  can 
be  easily  explained. 


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INDEX. 


PAGE 

Aberration 117 

Aerolites '. 175 

Aldebaran  ( Al  d6b'-a-ran)  . .  225 

Algol 242 

Altitude 27 

Amplitude 27 

Anaxagoras 8 

Anaximander 7 

Andromeda 233 

Antinous  and  Eagle 236 

Aphelion 18 

Apogee 123 

Apparent  motion 85 

Apsides  Ill 

Arcturus 805,  232 

Argo 239 

Aries 224 

Asteroids  154 

Astrology 13 

Astronomy,  Definition  of  1 

"  Antiquity  of 5 

Auriga  (Au  rl'-ga) 225 

Azimuth 27 

Baily's  Beads 141 

Bella'trix 228 

Berenice's  Hair  (Coma  Berenices') 231 

Betelgeuse  (Be-tel'-je-uze) 238 

Biela's  Comet 1  4,  ;i07 

Bode'sLaw 154 

Bootes  (Bo-O'-tes) 232 

Calendar,  The 269 

Canes  Venatici 231 

Canis  Major,  Canis  Minor 229 

Cancer 229 

Capricornus 236 

Cassiopeia  (Kas'-se-o-pe'-e-S) 220 

Castor  and  Pollux 227 

Celestial  Chemistry 258 

"        Longitude 30 

"        Measurements 271 

"        Pole 28 

"       Sphere 24 


PAGE 

Centaur 236 

Cepheus 219 

Ceres 155 

Ceius 226 

Chaldeans 6 

Chinese 6 

Chromosphere,  The 53 

Colures  (€o-liire' ;  plu.  €o-lflres') 27 

Comets 185 

Conjunction 60,  64 

"          Inferior 64 

"          Superior 64 

Constellations 209 

Copernican  System 14 

Cor  Caroli 231 

Corona  of  Sun 53,  141, 143,  303 

Cross,  The  Southern 239 

Crystalline  Spheres 8 

Cygni,No  61 204,241 

Cygnus 237 

Day  and  Night  87 

•'    Change  in  Length  of  .  98 

"    Civil,  The 267 

"    Sidereil 268 

"    Solar 264 

Declination    .   27, 29 

Dipper 216 

Diurnal  Motion 87 

Dolphin 237 

Draco •. 218 

Dry  Moon 131 

Earth 82 

"     Eccentricity  of  Orbit 91 

"     Rotation  of 86 

"     Rotundityof 85 

"     Yearly  Motion  of 91 

Earth-shine 127 

Eccentricity 57 

Eclipses 138 

Ecliptic 29,94 

"       Obliquity  of 29,  95 

"       Plane  of 58 


324 


INDEX. 


PAGE 

Ecliptic,  Poles  of 30 

Egvptiaus 8 

Ellipse 17,57 

Elongation -. .   . . .-    64 

Equinoxes 88,  30,  98 

"         Precession  of 104 

"         Vernal 104 

Equinoctial 27 

Eudosus 8 

Evening  Star 65,  70 

Faculae 50 

Fixed  Stars,  The 204 

Distance  of 204 

"  Motion  of 205,  261 

I?ame#  of 208 

"  Parallax  of 204 

Flames,  Solar 262 

Focus 17,  57 

Galaxy 253 

Galileo  (Ga-i-lee-o) 19 

Gemini  226 

Geocentric 64 

Gibbous 127 

Golden  Number 145 

Granules 50 

Gravitation 24 

Grecians 6 

Greek  Alphabet 208 

Gregorian  Calendar 269 

Hall,  Prof.  Asaph l?2-3 

Hare 228 

Harvest  Moon 130 

Heliocentric 64 

Hercules 233 

Herschel 170 

Herschel's  Theory 254 

Hipparchus 8 

Horizon 26 

Horoscope 13 

Hour  Circles 27 

Hyades 224 

Hydra 231 

Inferior  Planets 63 

Interior       '' 56 

Irradiation 123 

Job's  Coffin 237 

Jupiter 157 

Kepler 15 


PAGE 

Kepler's  Laws 15 

Kirchhoff's  Theory  (KirkTiof) 53 

Latitude,  To  Calculate  218.281 

Leo  229 

Libra 235 

Li'jrations 124 

Lisht.  Aberration  of . . .     117 

"      Refraction  of 118 

•'      Velocity  of 162 

Longitude,  To  Calculate 280 

Lunarians,  The 126 

Lyra  238 

Magellanic  Clouds 253 

Mars 150 

Mean  Day 264 

Mercury 71 

Meridian 26 

Meteoroids 1S2 

Meteors  175 

Metonic  Cycle 144 

Milk  Dipper 236 

Milky  Way 253 

Minor  Planets 154 

Mira 242 

Moon 122 

•'    Differential  Effect  of 148 

•'    Eclipse  of 145 

Morning  Star 65,  70 

Motion,  Apparent 85 

"        Diurnal 87 

"        of  Star 205,261 

Nadir 26 

Naos 239 

Nebular  Hypothesis 255 

Nebulae 246 

"       Spectraof 262 

Neptune 172 

Newton 21 

Nodes 58,  133 

Noon-mark 265 

North  Potar  Star 217 

Nucleus 53,  186 

Nutation 109 

Occultation 132 

Ophincus 834 

Opposition 67 

Orbit  of  Planets 57 

"            "      Solar  System 206 

Orion  (O-ri-on) 227 


I 


INDEX. 


325 


P&.GT: 

Parallax 119 

Annual 121 

Change  of  Solar ST9 

"      Horizontal 121 

"       Lunar 2T3 

"       Solar 3G,  121,275 

Pegasus  (Peg'-a-sus) 223 

Penumbra 51,139 

Perigee ' 123 

Perihelion 18 

Perseus 221 

Phases GO 

Photosphere 51 

Pisces 226 

Planets 55 

"      Are  they  inhabited  ? 61 

"       Definition  of 3 

"       Size  of 50 

Pleiades  (P16'-ya-dez) 235 

Polaris 217 

Polar  Star 217 

"     distance 29 

Precession  of  Equinoxes 104 

Procyon  (Pr6-9y-on) 229 

Protuberances,  Solar 53 

Ptolemaic  Theory 9 

Ptolemy 9 

Pythagoras 7 

Quadrature 03 

Refraction 112 

Regulus 229 

Retrograde  motion 05 

Right  ascension 29 

Rising  and  setting 86 

Sagittarius 236 

Saracens 11 

Saros 6,144 

Saturn 164 

Scintillation 207 

Scorpio 235 

Seasons 95 

Serpentarius 234 

Shooting  Stars 175 

Sidereal  Revolution    45,  69 

Sidereal  System 201 

Signs,  Zodiacal 31,  lOfi,  210 

Signs  and  Constellations  not  asreeiiiir  211 

Sirius 204,229,308 

Solar  System  35 

"  Motion  of 205 


PAGE 

Solar  timo 264 

Solstices 30,  98 

Space 24 

Spectra 258 

Spectrum  Analysis 258 

Spectroscope 259 

Spica 230 

S:ais,  The , 203 

"      Colored 241 

"      Distance  of 204 

"      Diunial  Orbits  of 89 

"       Double 239 

"      Number  of 200 

"      Proper  motion  of 205,261 

"      Size 207 

"      Temporary 243 

"      Variable 242 

Sun 36 

"    Change  in  form  and  place  of .  ...  114 

"    Diurnal  motion  of 87 

"    Eclipse  of 138 

'•    Heatof 54 

"    Path  of 94 

"    Protuberances  of o^i.  262 

"    Yearly  Path  of 94 

"    Spots 40 

Superior  Planets 63 

Syzygies 149 

SjTiodic  Revolution 45,  69 

Taurus 224 

"      Poniatowskii 234 

Thales 6 

Thuban 219 

Tides 147 

Time 263 

Top 109 

Transit 65 

Transit  of  Venus 277 

Triangles 224 

Twilight 116 

Twinkling 207 

Tycho  Brahe  (Bra  or  Bra) 15 

Umbra 138 

Uranus  (U'-ra-nns) 170 

Ursa  Major 315 

"     Minor 217 

Vega 217,238 

Venus 77 

"      Transit  of 277 

Vertical  Circle 26 


32»j 


INDEX. 


PAGE  I 

W-locit y  of  Light 168  }  Year.  Anomalistic  — 

Virgo 230  1      "      of  Astronomers 

Vulcan "1  I      '•      Sidereal 

Tropical 

Wet  Moon 131 

Wilson's  Theory 51 


FA6B 

....  268 
111,301 


268 


Year,  The 


Zeuith 26 

Zodiac  (Zo-di-ac) 31,210 

2e8  i  Zodiacal  Light  (Zo-dl'-ac-ai  i 196 


This  book  is  DUE  on  the  last  date  stamped  below 


3V7    19511 

.  -  t,  19641 


orm  L-9-15m-7,'32 


A    000  1 69  908     1 

Mathematical 

Sciences 

Library 


AUXILIARY 

.ST''CK 


M.72 


;/ 


OTIVEKSITY  of  CALIFORNIA 

^ub  ANGELKS 
LIBR-ARY 

DEPARTMENT 
t  OF 


ASTRONOMY 


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